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• Removes dependence on outdated version of sphinx_rtd_theme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 [WIP] Add Github workflow: "Pull Request Docs Check" #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
  • Publish docs update on merge No, do this separately. See Automatically publish updated docs #2839 and samr-test-20241003 #2812.
• Could rid us of the need for Docker and doc-builder (would require more testing; probably a different PR)

Blocked by ESMCI/doc-builder#5

Contributors other than yourself, if any: None

CTSM Issues Fixed (include github issue #):

• Resolves Update doc-builder #2818

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.

[samsrabin]

After merge of #2982:

• Update python/conda_env_ctsm_py.txt to use doc/requirements.txt
• Update GitHub workflows to use ctsm_pylib

[samsrabin]

Blocked by ESMCI/doc-builder#5:

• Update doc-builder in .gitmodules to point to the canonical version

[samsrabin]

• Add Github workflow testing build-docs with Docker container.