Add note on using Mamba instead of conda

doc/htmldoc/installation/conda_forge.rst

@@ -3,6 +3,13 @@
 Conda forge install
 ===================
 
+.. note:: 
+
+   If you encounter problems installing the NEST conda package and 
+   environment, we recommend using Mamba (https://mamba.readthedocs.io). 
+   Mamba has the advantage of installing conda packages and 
+   environments more quickly and can be used as a complete drop-in replacement for conda.
+
 1. To keep your conda setup tidy, we recommend that you install NEST into
    a separate `conda environment <https://docs.conda.io/projects/conda/en/latest/user-guide/tasks/manage-environments.html>`_
    together with Python packages that you will use when working with NEST;

doc/htmldoc/installation/conda_tips.rst

@@ -3,10 +3,16 @@
 Tips for installing NEST with conda
 ===================================
 
+.. note:: 
+
+   If you encounter problems installing the NEST conda package and 
+   environment, we recommend using Mamba (https://mamba.readthedocs.io). 
+   Mamba has the advantage of installing conda packages and 
+   environments more quickly and can be used as a complete drop-in replacement for conda.
+
 This page provides a series of recommendations for installing pre-built NEST with
 conda or to set up conda environments for building NEST and NEST documentation.
 
-
 Basic conda setup
 -----------------
 

doc/htmldoc/installation/condaenv_install.rst

@@ -3,6 +3,13 @@
 Install from source in a conda environment
 ==========================================
 
+.. note:: 
+
+   If you encounter problems installing the NEST conda package and 
+   environment, we recommend using Mamba (https://mamba.readthedocs.io). 
+   Mamba has the advantage of installing conda packages and 
+   environments more quickly and can be used as a complete drop-in replacement for conda.
+
 * Create a conda environment from the `environment.yml <https://github.com/nest/nest-simulator/blob/master/environment.yml>`_ file.
   We recommend specifying a dedicated location (``-p <path/to/conda/env>``) for your environment.
   See the `conda documentation <https://docs.conda.io/projects/conda/en/latest/user-guide/tasks/manage-environments.html#specifying-a-location-for-an-environment>`_