Build documentation with mkdocs

[smokestacklightnin]

This PR builds the documentation using mkdocs.

The following is included in this PR:

• Add mkdocs.yml configuration along with associated javascript and stylesheets
• Automatically render API docs using mkdocstrings
• Remove deprecated API docs stored as markdown files in favor of automatically generated API docs. This is advantageous for several reasons, including that developers no longer need to update both docstrings and separate markdown files to document their code.
• Add modules that should be included to __all__ list in appropriate __init__.py files
• Move example notebooks to documentation directory so they are rendered in docs
• Add a deployment workflow for the repository
• Fix formatting of code and buttons
• Fix some broken links
• Use README.md as homepage by creating a softlink to it from the documentation folder

In order to deploy the documentation, a maintainer will need to enable publishing from a branch on this repo for the gh-pages branch after this PR is merged.

Here is a preview of the new documentation

[peytondmurray]

A few changes mentioned below, but also:

Can we have a workflow that also builds (but doesn't deploy) the docs trigger when a PR is made? With the way it currently is currently building on push to master, contributors can easily break documentation without knowing it.

[peytondmurray]

Can these be moved into a dedicated optional dependency? pip install '.[docs]' or similar so that the dependencies can actually be tracked.

[smokestacklightnin]

Done

[smokestacklightnin]

Can we have a workflow that also builds (but doesn't deploy) the docs trigger when a PR is made? With the way it currently is currently building on push to master, contributors can easily break documentation without knowing it.

Made this change, mirroring what we've done for the other repos we're working on.

[peytondmurray]

Hi @hassler-google, is there anything else I can do to help push this forward? It would be great to get this merged.

[peytondmurray]

Wow, this one was a lot of work. Thanks for this!

One request: can the docs_dir be the default docs/? Or is there some reason to have them be under g3doc?

[smokestacklightnin]

One request: can the docs_dir be the default docs/?

@peytondmurray Done