[Doc] Upgrade build tooling

[peytondmurray]

Why are these changes needed?

This PR implements build tooling changes needed for the doc build system upgrade.

• Bump RTD python version to 3.11, which is ~40% faster than 3.10
• Add a few build flags:
  • -W: Turn warnings into errors during the build
  • -j auto: Enable parallel builds, significantly improving the time it takes to build the docs
• Bump the versions of the build system in requirements-doc.txt
• Remove sphinx-version-warning - we weren't using it, it's unmaintained, and the JS was throwing errors upon page load
• Remove sphinx-tabs, because pydata-sphinx-theme uses sphinx-design, which has a tab widget already. Having two tab widgets is a recipe for conflicting styles; additionally, sphinx-tabs has a bug where the js assets for tabs are injected on every page even if no tabs are being used on that page; see sphinx-tabs assets are not removed from pages where tabs aren't being used executablebooks/sphinx-tabs#184
• Remove sphinx-external-toc as it is incompatible with the latest versions of sphinx. It's not needed here, and has not seen much development in the past year. _toc.yml is removed as a result of this.
• Switch to pydata-sphinx-theme, which is the theme on which sphinx-book-theme is based. It's currently actively maintained, and has seen recent releases with big improvements to accessibility. It also comes with a lot of what we want right out of the box, and using it removes a dependency from our docs build system. I think I've talked this decision at length now to multiple people, but if there are still any questions about this choice I'm happy to discuss further.

Related issue number

Partially addresses ray-project#37944.

PR 1/x targeting ray-project#41115.

Tests may not pass here because some css and js assets and templates will be committed in a separate PR.

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[maxpumperla]

nice!

[maxpumperla]

This is great, we really needed that clean-up.

Can be done later, but can we move the parse_navbar_config etc. to e.g. custom_directives.py or so? conf.py is the one file most of our devs have to have a look at occasionally, and I don't want to blow it up too much.

[peytondmurray]

Can be done later, but can we move the parse_navbar_config etc. to e.g. custom_directives.py or so? conf.py is the one file most of our devs have to have a look at occasionally, and I don't want to blow it up too much.

Yep, good idea. I'll make a checklist for this kind of cleanup in the target branch for this PR, and do this before we merge ray-project#41115.