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