Add readthedocs support

[wllgrnt]

Motivation and Context

Once merged, this will allow the Sphinx docs in docs/ (and the docstrings) to be built and served at typed-python.readthedocs.io. With an appropriate webhook the docs would be rebuilt automatically with every commit to dev but that's a separate discussion.

Apologies for the many commits - the RTD/sphinx-apidoc documentation is sparse, and learning which shells RTD and Make like to use, and which of those support the globstar operator, was non-trivial.

How Has This Been Tested?

This branch's docs are viewable at https://typed-python.readthedocs.io/en/will_grant-add_markdown_rtd_support/. Will delete once merged.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.

[wllgrnt]

(We can hang fire on this until the docs are in a better state)

[braxtonmckee]

This looks good. Can we squash it down to one commit and then i'll merge it?