[air] Refactor Docs for re-emphasizing libraries (3/n)

[richardliaw]

Why are these changes needed?

This PR makes major structural changes to the TOC, as previously agreed upon (cc @ericl).

In future PR, we will need to:

• Remove references to Ray AIR as a concept
• Clean up our examples bloat
• Refactor the API page
• Make sure user journeys are still successful

Related issue number

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• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
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• 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 :(

[angelinalg]

Two suggestions for the TOC that could make the organization cleaner, if it makes technical sense to you.

[angelinalg]

Suggest moving Computer Vision one level up because it is a Use Case.

[angelinalg]

Suggest moving this content into Ray for ML Infrastructure to reduce the number of levels.

[ericl]

The only thing I feel is a bit weird is that "Use cases" doesn't have the dropdown caret, but I noticed we fixed this for the "User Guides" subsections now. So it seems inconsistent.

If there is the concern users would click the caret, bypassing the landing page, how about making the Caret greyed out / unclickable but visible, or making it visible but clicking it goes to the use cases landing page still?

[angelinalg]

@ericl, I think it's fine to add a caret for Use Cases. @richardliaw, you just need to add Use Cases to the stringList on line 38 of this file: /ray/doc/source/_static/js/custom.js