Add code documentation

[jbheard]

We currently put documentation for how to use the project in /docs. It would be nice to also have documentation on our internal functions, classes, etc. This has a number of benefits, including:

• new contributors will have an easier time learning the code
• misunderstanding when & how to use things will be far less common

If we do this, I think the best course of action is to have a github action that generates markdown & commits markdown files based on docstrings in the code.

We could write documentation separately from the code, but that has some drawbacks:

• People are less likely to write documentation when it is separate from the code
• Code changes fast, and is modified by multiple people, so docs will quickly become outdated

[jbheard]

This was resolved with the Build Documentation action I built.