[Doc] Refactor example gallery to remove temporary flash

[peytondmurray]

Why are these changes needed?

This PR reworks the docs example gallery to fix an annoying transient state that pops up when the example gallery first loads, before the js that overrides a bunch of default sphinx styles is executed and replaces the content.

This is achieved by directly inserting a link to a stylesheet into the example gallery file instead of loading it via js. This applies styles on first page load rather than whenever js gets around to executing. Other minor changes:

• Removed a bit of the script that runs on the example gallery that removes content from the site navigation bar on the left of the page. It's unnecessary to modify the DOM; we can just hide these nodes with css, and should speed up rendering.
• Removed a wildcard import in conf.py; linters don't like these and it's less clear than explicit imports.

Related issue number

Closes #37216.

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 :(

[simran-2797]

Awesome! LGTM.