[Doc] Optimize build time

[peytondmurray]

Why are these changes needed?

This PR optimizes the build time of the docs-build-system-upgrade branch.

It does this by generating the sidebar HTML once (per worker) and then reusing it on every page, bypassing pydata-sphinx-theme's sidebar construction machinery. To maintain the collapsed/expanded state of the sidebar dropdown items, we use the browser's local storage, which is restored on each page load.

• Moves the custom directives in conf.py into custom_directives.py at the request of @maxpumperla
• Removes reference to document that doesn't exist
• Add make show command, which used http.server to serve docs built locally
• Removes code for making "galleries", which we aren't using in the new docs build system

Related issue number

Partially addresses ray-project#37944.

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 :(