docs: initial doc generation using Sphinx

[rickeylev]

This lays the groundwork for using Sphinx to generate user-facing documentation and
having it published on readthedocs. It integrates with Bazel Stardoc to generate
MyST-flavored Markdown that Sphinx can process.

There are 4 basic pieces that are glued together:

1. sphinx_docs: This rule invokes Sphinx to generate e.g. html, latex, etc
2. sphinx_stardoc: This rule invokes Stardoc to generate MyST-flavored Markdown
   that Sphinx can process
3. sphinx_build_binary: This rule defines the Sphinx executable with any necessary
   dependencies (e.g. Sphinx extensions, like MyST) to process the docs in (1)
4. readthedocs_install: This rule does the necessary steps to build the docs and
   put them into the location the readthedocs build process expects. This is basically
   just cp -r, but its cleaner to hide it behind a bazel run command than have
   to put various shell in the readthedocs yaml config.

• Bump Bazel 6 requirement: 6.0.0 -> 6.20. This is necessary to support
  bzlmod and Stardoc.

Work towards #1332, #1484

[aignas]

Really interesting, thanks for sharing this!

[rickeylev]

Ok, I've cleaned this up enough to be ready for an initial review. This PR just lays the groundwork -- I'm not writing the complete porting of docs over to RTD in this PR.

What I'm going to do is:

1. Land this PR
2. Reconfigure RTD to read from a new branch, "rtd-testing". This is where I'll push commits to the RTD build process and get it working
3. Land another PR on main with the changes from (2). At this point, RTD will be building and publish main and our tagged versions.
4. Enable RTD building for PRs
5. Then we can port our plain stardoc md/html docs over to Sphinx/RTD. This will require various changes to docstrings etc so that things group and render nicely.

[aignas]

In general this looks great! looking forward to better docs. Maybe linking #1332 should be mentioned in the PR description?

[rickeylev]

linking #1332 should be mentioned in the PR description?

Done