Add community page to docs

[teddygroves]

This change addresses issue #548 by adding a community section to the documentation.

Sidenote - I had to install the following packages after running pip install -e '.[docs]' in order to get make html to work: jupyter nbsphinx sphinx-copybutton pydata-sphinx-theme. I think the correct docs requirements are in docsrc/env.yml, but I don't know how to install them from the file. Can anyone could tell me the best way to get set up for editing cmdstanpy's docs?

Submission Checklist

• Run unit tests
• Declare copyright holder and open-source license: see below

Summary

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company):

Teddy Groves

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)

[WardBrian]

docsrc/env.yml is a conda environment file which is used by readthedocs. I don't think we have anything set up for pip to install the documentation dependencies at the moment

[WardBrian]

This looks great. Thanks!