Investigate MyST for doc building

[jklaise]

MyST is a new Markdown format emulating the functionality of RST and fully compatible with Sphinx. We could use this instead of m2r (unmaintained) or recommonmark to support .md file parsing in docs. This could allow a more streamlined workflow of writing docs in Markdown (instead of RST) but maintain flexibility and avoid Sphinx warnings parsing vanilla Markdown files. https://github.com/executablebooks/myst-parser

They also support parsing Jupyter notebooks via https://github.com/executablebooks/myst-nb which could be a replacement for nbsphinx, but may lack some features we currently use (e.g. executablebooks/MyST-NB#297). It would be nice to remove the non-python pandoc dependency that is required by nbsphinx.

[ascillitoe]

This is largely addressed by #482.

@jklaise do we want to close this, or keep open (or open new issue) to explore myst-nb? Looks like myst-nb isn't moving towards nbsphinx-link type support anytime soon, so we would have to think about build time copying around of example notebooks...

[jklaise]

Yea I think we're good to close this as we've achieved the main goals. We can keep monitoring progress of myst-nb for the future.

It may be helpful to convert existing .ipynb documentation files into .md but probably a new issue would better describe it.