Add documentation with MkDocs

[smokestacklightnin]

This PR adds guide and API documentation using MkDocs.

Depending on how much time the decision makers want me to spend, I can fix MarkDown formatting issues. Until then, the docs contained herein are still usable.

Only modules written in Python are included. Modules coming from protocol buffers are not included.

A preview can be found here.

[peytondmurray]

As a matter of preference I'd vote to preserve the nesting of tfx_bs.public.tfxio, i.e. make separate pages for each submodule. As it stands, mkdocstrings flattens the module hierarchy, and that page ends up being suuuuuper long: https://smokestacklightnin.github.io/tfx-bsl/api_docs/python/tfxio/. To address that I think it would be nice to make just a separate page for each module - would that be okay?

[smokestacklightnin]

As a matter of preference I'd vote to preserve the nesting of tfx_bs.public.tfxio, i.e. make separate pages for each submodule. As it stands, mkdocstrings flattens the module hierarchy, and that page ends up being suuuuuper long: https://smokestacklightnin.github.io/tfx-bsl/api_docs/python/tfxio/. To address that I think it would be nice to make just a separate page for each module - would that be okay?

I had actually included the wrong modules, but it is fixed now. Now only the correct classes for the correct module are included, and the page is not nearly as long.