replace mailing list with GitHub Discussions in documentation (#1902)

doc/docs/Acknowledgements.md

@@ -5,7 +5,7 @@
 Authors
 -------
 
-Meep originated as part of graduate research at [MIT](https://en.wikipedia.org/wiki/Massachusetts_Institute_of_Technology) in the mid 2000s with initial contributions by [Steven G. Johnson](http://math.mit.edu/~stevenj/), [Ardavan Oskooi](http://ab-initio.mit.edu/~oskooi/), [David Roundy](http://physics.oregonstate.edu/~roundyd/), [Mihai Ibanescu](https://www.linkedin.com/in/mihai-ibanescu-2b147825/), and [Peter Bermel](http://web.ics.purdue.edu/~pbermel/). The project has been under continuous development for nearly 20 years. Currently, the Meep project is maintained by an active developer community on [GitHub](https://github.com/NanoComp/meep). [Christopher Hogan](https://github.com/ChristopherHogan) and [M.T. Homer Reid](http://homerreid.dyndns.org/) lead the development of the [Python interface](Python_User_Interface.md), [mode-decomposition feature](Python_Tutorials/Mode_Decomposition.md), and [GDSII import routines](Python_Tutorials/GDSII_Import.md). M.T. Homer Reid and [Alec Hammond](https://github.com/smartalecH/) developed the [adjoint solver](Python_Tutorials/Adjoint_Solver.md). [Alex Cerjan](http://www.alexcerjan.com/) assisted with adding support for saturable absorption via [multilevel atomic gain media](Materials.md#saturable-gain-and-absorption). Alec Hammond developed the [visualization module](Python_User_Interface.md#data-visualization). [Yidong Chong](http://www1.spms.ntu.edu.sg/~ydchong/bio.html) and Alex Cerjan added support for [gyrotropic media](Materials.md#gyrotropic-media). [Andreas Hoenselaar](https://github.com/ahoenselaar) contributed to several performance enhancements.
+Meep originated as part of graduate research at [MIT](https://en.wikipedia.org/wiki/Massachusetts_Institute_of_Technology) in the mid 2000s with initial contributions by [Steven G. Johnson](http://math.mit.edu/~stevenj/), [Ardavan Oskooi](http://ab-initio.mit.edu/~oskooi/), [David Roundy](http://physics.oregonstate.edu/~roundyd/), [Mihai Ibanescu](https://www.linkedin.com/in/mihai-ibanescu-2b147825/), and [Peter Bermel](http://web.ics.purdue.edu/~pbermel/). The project has been under continuous development for nearly 20 years. Currently, the Meep project is maintained by an active developer community on [GitHub](https://github.com/NanoComp/meep). [Christopher Hogan](https://github.com/ChristopherHogan) and [M.T. Homer Reid](http://homerreid.dyndns.org/) lead the development of the [Python interface](Python_User_Interface.md), [mode-decomposition feature](Python_Tutorials/Mode_Decomposition.md), and [GDSII import routines](Python_Tutorials/GDSII_Import.md). M.T. Homer Reid and [Alec Hammond](https://github.com/smartalecH/) developed the [adjoint solver](Python_Tutorials/Adjoint_Solver.md). [Alex Cerjan](http://www.alexcerjan.com/) assisted with adding support for saturable absorption via [multilevel atomic gain media](Materials.md#saturable-gain-and-absorption). Alec Hammond developed the [visualization module](Python_User_Interface.md#data-visualization). [Yidong Chong](http://www1.spms.ntu.edu.sg/~ydchong/bio.html) and Alex Cerjan added support for [gyrotropic media](Materials.md#gyrotropic-media). [Andreas Hoenselaar](https://github.com/ahoenselaar) contributed to several performance enhancements. [Krishna Gadepalli](https://github.com/kkg4theweb) added support for checkpointing the simulation state.
 
 Referencing
 -----------

doc/docs/Build_From_Source.md

@@ -4,7 +4,7 @@
 
 The main effort in installing Meep lies in installing the various dependency packages. This requires some understanding of how to install software on Unix systems.
 
-**Note**: When downloading the source code from the [Meep Releases](https://github.com/NanoComp/meep/releases) page on Github, be sure to download the `meep-X.Y.Z.tar.gz` release file, *not* the `vX.Y.Z.tar.gz` file linked under "Source Code".  The latter is autogenerated by Github and is equivalent to the files you would get from a `git clone`, which require running a `autogen.sh` script and special tools as explained under [Meep for Developers](Build_From_Source.md#meep-for-developers) below.  The `meep-X.Y.Z.tar.gz`, in contrast, includes all pre-generated source files, though you still need Unix compiler tools.
+**Note**: When downloading the source code from the [Meep Releases](https://github.com/NanoComp/meep/releases) page on Github, be sure to download the `meep-X.Y.Z.tar.gz` release file, *not* the `vX.Y.Z.tar.gz` file linked under "Source Code".  The latter is autogenerated by Github and is equivalent to the files you would get from a `git clone`, which require running a `autogen.sh` script and special tools as explained under [Meep for Developers](Build_From_Source.md#meep-for-developers) below.  The `meep-X.Y.Z.tar.gz`, in contrast, includes all [pre-generated source files](https://www.gnu.org/software/automake/manual/html_node/CVS.html), though you still need Unix compiler tools.
 
 It is also possible to install Meep on Windows systems. For Windows 10, you can install the [Ubuntu 16.04](https://www.microsoft.com/en-us/p/ubuntu-1604-lts/9pjn388hp8c9) or [18.04](https://www.microsoft.com/en-us/p/ubuntu/9nblggh4msv6) terminal as an app (via the [Windows Subsystem for Linux](https://docs.microsoft.com/en-us/windows/wsl/about) framework) and then follow the instructions for [obtaining the Conda packages](Installation.md#conda-packages) (recommended) or [building from source](Build_From_Source.md#building-from-source). For Windows 8 and older versions, you can use the free Unix-compatibility environment [Cygwin](http://www.cygwin.org/) following these [instructions](http://novelresearch.weebly.com/installing-meep-in-windows-8-via-cygwin.html).
 

doc/docs/FAQ.md

@@ -19,7 +19,7 @@ Meep was originally developed as part of graduate research at MIT. The project h
 
 ### Where can I ask questions regarding Meep?
 
-There is a public [mailing list](http://ab-initio.mit.edu/cgi-bin/mailman/listinfo/meep-discuss) for users to discuss issues pertaining to setting up simulations, post-processing output, installation, etc. A useful place to start is the [list archives](https://www.mail-archive.com/[email protected]/) which includes all postings (6500+) since 2006 spanning a variety of topics. Bug reports and new feature requests should be filed as a [GitHub issue](https://github.com/NanoComp/meep/issues). However, do not use issues as a general help desk if you do not understand something (use the mailing list instead).
+The [Discussions](https://github.com/NanoComp/meep/discussions) page on GitHub can be used to ask questions regarding setting up simulations, analyzing results, installation, etc. Bug reports and new feature requests should be filed as an [issue](https://github.com/NanoComp/meep/issues) on GitHub. However, do not use issues as a general help desk if you do not understand something (use the Discussions instead).
 
 ### How can I contribute to the Meep project?
 

doc/docs/Materials.md

@@ -96,7 +96,7 @@ For example, suppose you want to simulate a medium with $\varepsilon = 3.4 + 0.1
 
 You can also use the $\sigma_D$ feature to model the [attenuation coefficient](https://en.wikipedia.org/wiki/Attenuation_coefficient) $\alpha$ (units of e.g. dB/cm) obtained from experimental measurements (i.e., ellipsometry). This involves first [converting $\alpha$ into a complex refractive index](https://en.wikipedia.org/wiki/Mathematical_descriptions_of_opacity#Complex_refractive_index) (which is then converted into a complex permittivity) with imaginary part given by $\lambda_0\alpha/(4\pi)$ where $\lambda_0$ is the vacuum wavelength.
 
-**Note**: the "conductivity" in Meep is slightly different from the conductivity you might find in a textbook, because for computational convenience it appears as $\sigma_D \mathbf{D}$ in our Maxwell equations rather than the more-conventional $\sigma \mathbf{E}$; this just means that our definition is different from the usual electric conductivity by a factor of $\varepsilon$. Also, just as Meep uses the dimensionless relative permittivity for $\varepsilon$, it uses nondimensionalized units of 1/*a* (where *a* is your unit of distance) for the conductivities $\sigma_{D,B}$. If you have the electric conductivity $\sigma$ in SI units and want to convert to $\sigma_D$ in Meep units, you can simply use the formula: $\sigma_D = (a/c) \sigma / \varepsilon_r \varepsilon_0$ where *a* is your unit of distance in meters, *c* is the vacuum speed of light in m/s, $\varepsilon_0$ is the SI vacuum permittivity, and $\varepsilon_r$ is the real relative permittivity.
+**Note**: the "conductivity" in Meep is slightly different from the conductivity you might find in a textbook, because for computational convenience it appears as $\sigma_D \mathbf{D}$ in our Maxwell equations rather than the more-conventional $\sigma \mathbf{E}$; this just means that our definition is different from the usual electric conductivity by a factor of $\varepsilon$. Also, just as Meep uses the dimensionless relative permittivity for $\varepsilon$, it uses nondimensionalized units of 1/$a$ (where $a$ is your unit of distance) for the conductivities $\sigma_{D,B}$. If you have the electric conductivity $\sigma$ in SI units of S/m (S is siemens) and want to convert to $\sigma_D$ in Meep units, you can simply use the formula: $\sigma_D = (a/c) \sigma / (\varepsilon_r \varepsilon_0)$ where $a$ is your unit of distance in *meters*, $c$ is the vacuum speed of light in m/s, $\varepsilon_0$ is the SI vacuum permittivity, and $\varepsilon_r$ is the real relative permittivity. The quantity $a/c$ in this equation is the conversion factor for frequency in SI units (s$^{-1}$ or Hz) to frequency in Meep units ($c/a$).
 
 Nonlinearity
 ------------

doc/docs/index.md

@@ -50,9 +50,11 @@ For a list of topics, see the left navigation sidebar. For new users, the most i
 
 This documentation is for the master branch of the [source repository](Download.md#github-source-repository). Note that certain features described in this documentation may therefore be *unavailable* if you are using a [tagged release](https://github.com/NanoComp/meep/releases).
 
-### Mailing Lists
+### Discussion Forum
 
-Subscribe to the read-only [meep-announce mailing list](http://ab-initio.mit.edu/cgi-bin/mailman/listinfo/meep-announce) to receive notifications of updates and releases. Subscribe to the [meep-discuss mailing list](http://ab-initio.mit.edu/cgi-bin/mailman/listinfo/meep-discuss) for discussions regarding using Meep. The [meep-discuss archives](https://www.mail-archive.com/[email protected]/) includes all postings since 2006 spanning a large number and variety of discussion topics related to installation, setting up simulations, post-processing output, etc. The list archives can also be accessed using a [newsgroup reader](https://en.wikipedia.org/wiki/List_of_Usenet_newsreaders) via the NNTP interface address: `news.gmane.io/gmane.comp.science.electromagnetism.meep.general`.
+For questions regarding setting up simulations, analyzing results, installation, etc., use the [Discussions](https://github.com/NanoComp/meep/discussions) page on GitHub.
+
+As an additional resource, archives of the [meep-discuss mailing list](https://www.mail-archive.com/[email protected]/), which is no longer active, are available which include postings from 2006-2021. These archives can also be accessed using a [newsgroup reader](https://en.wikipedia.org/wiki/List_of_Usenet_newsreaders) via the NNTP interface address: `news.gmane.io/gmane.comp.science.electromagnetism.meep.general`.
 
 ### Bug Reports and Feature Requests