[Doc] Use autodoc_mock_imports to mock deps for sphinx builds (#38817)

[GeneDer]

This PR is the first in a series needed to update Sphinx to the latest version. In this PR, we

• Remove a lot of external dependencies that aren't necessary for building the docs. The one exception to this is tune-sklearn, a project owned by ray-project which actually does have documentation hosted on the Ray docs site.
• External dependencies are now mocked out using Sphinx's autodoc_mock_imports mechanism, which is used by both autodoc and autosummary. The old module mocking mechanism has been removed in favor of this. The one exception to this is packaging.version.Version: Ray currently uses Version to modify some behaviors depending on the version of certain dependencies that the user has installed.

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Why are these changes needed?

To ensure docs on the release branch are up-to-date

Related issue number

Pick of #38817

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 :(

[zhe-thoughts]

Clean docs change; qualifies for picking