Why are these changes needed?

This PR adds/modifies/removes assets and templates for the new build system.

• Removes LandingPageBG.jpg, which wasn't being correctly loaded in the index.html page, and which doesn't work well on dark theme anyway
• Modified custom.css to include only the side-wide CSS rules we need - this was done line-by-line, so a lot of unnecessary rules have been removed, mostly because don't apply to the new pages anymore, or they don't make sense to do with the updated theme. Note that the ray-blue CSS variable is stored globally here!
• Modified custom.js:
  • Previously, a part of this script would check the current URL on each page load to see if we were on the example gallery page. If we were, it would dynamically remove DOM content from the sidebar and replace it with a custom sidebar. This is now handled with templating at sphinx build time, so this isn't needed anymore.
  • Part of this script looked for fixed height images and adjust the height of containing elements to make card grids pretty. This is now handled with pure CSS.
• docsearch.js has been removed. We have builtin search now; if we don't like it, we can replace it later on.
• top-navigation.js has been removed; the top bar is rendered with a template at build time rather than with a script.
• breadcrumbs.html has been removed, I don't think it was being used. pydata-sphinx-theme has a breadcrumbs widget builtin.
• hljs has been updated to 11.9.0.
• The CSAT widget is being rendered without overriding footer-content.html, so this is no longer needed.
• The default toctree sidebar is currently being used, so sbt-sidebar-nav.html is no longer needed. Also, we're not using sphinx book theme anymore.

Related issue number

Partially addresses ray-project#37944.
Depends on #2.
PR 6/x targeting ray-project#41115.

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 :(