Update docs infrastructure

[samsrabin]

Description of changes

• Removes dependence on outdated version of sphinx_rtd_theme
• Enables embedding of Markdown docs in .rst files,
• Adds Github workflow job to test documentation build whenever a PR is submitted with a branch from a fork that changes docs/
  • Allows closure of PR #2499
  • Allows deletion of test: docs label
• Adds Github workflow jobs to run for any PR with a branch from this repo that changes docs/:
  • Build and preview documentation on PR submission, update, or merge (untested)
  • Publish docs update on merge (untested)
• Could rid us of the need for Docker and doc-builder (would require more testing; probably a different PR)

Specific notes

Note that this PR tests the workflow jobs that are run on PRs of branches from forks. Specifically, the doc building is tested, but nothing is previewed or published. That only happens for PRs of branches from this canonical ESCOMP/CTSM repo; see #2812 for testing of that.

Contributors other than yourself, if any: None

CTSM Issues Fixed (include github issue #): None

Are answers expected to change (and if so in what way)? The way users switch between documentation versions will be different.

Any User Interface Changes (namelist or namelist defaults changes)? No

Testing performed, if any: Builds locally with no errors or warnings. Hopefully the version switching isn't broken—it looks broken locally, but I don't know if that's just an issue of it being local.

[ekluzek]

@billsacks it seems like this would be good for you to review some of this. Just the doc building part of this that is. So ignore the .github workflow part and the python directory part, and just look at the changes under the doc directory that are outside of the users-guide and tech-note changes.

Let us know if that's something you could do. Thanks for considering.

[samsrabin]

File issue:

• [x] Fix links that are messed up because they assume capital letter in filename. E.g., Running-with-custom-crop-calendars.rst in doc/source/users_guide/running-special-cases/index.rst. That page path seems to get produced as running-with-custom-crop-calendars.html instead of Running-with-custom-crop-calendars.html, so the link is broken.
• [x] Add a GitHub workflow to fail if any .rst or .md filename in (or linked in) the docs has a capital letter.

Added as #3121.

[samsrabin]

Thanks, @billsacks!