docs: make readthedocs render a bit nicer and port docs over to Sphinx

[rickeylev]

This makes the Sphinx-based docs hosted on readthedocs render a bit more nicely, fixes a few issues, and adds some features to //sphinxdocs

This also moves all the docs onto Sphinx, deleting the checked-in documentation.

Doc fixes/improvements:

• Ports various docs over to Sphinx pages. They're split out from the readme file.
• Version RTD is building is reflected in the docs
• Fixes some references to github files
• Includes the custom CSS file that styled the api docs
• Removes -Q from doc building; all warnings should be fixed now
• Added Bazel inventory file. Bazel doesn't provide one, but we can manually provide on
  and still use intersphinx functionality.
• Added gh-path custom role. This is a shortcut for writing the whole github URL.
• Sets the primary domain to None. The default is py, which we don't use much of, so it
  just results in confusing crossref errors.
• Enable nitpicky mode to catch more errors.
• Remove the starlark marker from codeblocks; that name isn't recognized by Sphinx.
  The highlighting is still sufficient.
• Adds a glossary

Sphinxdocs improvements:

• Added a flag to pass along arbitrary -D args to the Sphinx invocations. This allows
  e.g., the version setting of the docs to be set on the command line from the
  READTHEDOCS_VERSION environment variable
• Added inventory file generation. These are files that allow referencing external
  projects using intersphinx.
• sphinx_stardocs have their public load path set as their page title. This groups the
  API docs more naturally by file. The path can be customized.
• sphinx_stardocs can have a footer specified for generated pages. This allows easily
  added a list of link labels for easy re-use.
• readthedocs_install now tries harder to find an open port
• The conf.py file is moved into the generated sources directly. This was done because some
  config settings are relative to the conf.py file, which was being placed one directory
  above the regular sources.

Fixes #1484, #1481

[rickeylev]

This can be previewed at: https://rules-python.readthedocs.io/en/latest/

[rickeylev]

Hm, you know, I think i'll just delete the old stuff in docs/ in this PR, too. That'll avoid any odd rendering issues with myst-markdown in the bzl code, and fix the issue with bazel's downstream testing. I'll make sure to copy over any of the manual merging like what py_console_script_binary was doing