docs: use sphinxdocs for docgen

[rickeylev]

This switches the doc generation over to use rules_python's sphinxdocs. Sphinxdocs
is a set of rules for running Sphinx with special integrations for Starlark
doc generation.

The main benefits of this switch are:

• Sphinx is entirely run through Bazel (instead of a bespoke shell script
  to glue together outputs of Bazel invocations)
• Sphinxdocs comes with a Sphinx plugin that allows defining Starlark objects
  as first-class objects to Sphinx. These are usually generated by the Stardoc
  integration, but can also be manually defined to document e.g. targets or flags.
• Sphinxdocs supports defining "typedefs", which is a way to group a collection of
  objects and have them rendered similar to how a class is rendered in other languages.
  Part of what this change does is update all the subject "classes" to render as
  "typedefs".
• Cross references are easier and more automatic. Because Sphinx understands the
  objects being defined, cross references have a more natural, concise, and expressive
  notation.
• Sphinxdocs has helpers for interactively running docs for easier edit/run cycles
• Sphinxdocs has helpers for using Bazel to build the docs as part of readthedocs
  workflows (this ensures what is built locally matches what is built by RTD).