[Doc] Theme revamp: styling changes

[peytondmurray]

Why are these changes needed?

This PR modifies a few global docs styles and makes index.html use the same pygments-based code highlighting as elsewhere in the docs.

• Instead of inserting the raw html for index.html using sphinx, I made the index page use its own template. This allows us to pass in a pygments highlighting function to the HTML context, which produces the same code highlighting used elsewhere in the docs.
• Renamed splash.css -> index.css, splash.js -> index.js

Related issue number

Closes #42052.

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 :(

[aslonnie]

changes mostly look good.

please address the string quoting styling consistency issue.

[peytondmurray]

Thanks for the feedback, should be ready for merge pending approval from @simran-2797.