Create Sphinx documentation to host these docs

[willingc]

Create Sphinx documentation and add to ReadTheDocs

[westurner]

• https://www.sphinx-doc.org/en/master/usage/markdown.html
• https://docs.readthedocs.io/en/stable/intro/getting-started-with-sphinx.html#using-markdown-with-sphinx
• https://docs.readthedocs.io/en/stable/intro/getting-started-with-mkdocs.html

[westurner]

• https://github.com/quobit/awesome-python-in-education/blob/master/.travis.yml
  • https://github.com/dkhamsing/awesome_bot "Validate links in awesome projects"
• sphinx make linkcheck does something similar
  • https://www.sphinx-doc.org/en/master/man/sphinx-build.html