[docs] AIR docs changes todo list

[richardliaw]

• [x] API pages need to be moved to the proper sections, out of ray-air @matthewdeng
  • In particular, the https://docs.ray.io/en/master/ray-air/api/api.html page should be removed, and all underlying pages should be removed as well. Any APIs in these underlying pages should be migrated to their correct positions in corresponding libraries.
  • [x] started PR -https://github.com/ray-project/ray/pull/38427
  • [x] done in https://github.com/ray-project/ray/pull/38632
• [x] remove AIR references (first pass done, need to do a final pass) @richardliaw
• [x] Move stuff out of Preprocessors doc page (@richardliaw)
  • [x] started pr (#38348)
• [x] remove examples @richardliaw
  • [x] started pr - https://github.com/ray-project/ray/pull/38374
• [x] Refactor rest of examples (P1?) @matthewdeng
  • This mainly means that we should remove as many examples in ray-air/examples/ as possible, into train/examples and such
  • [x] #39298
• [ ] Remove dead code and dead tests (NEED OWNER)
• [ ] Final pass @richardliaw

[anyscalesam]

The Train GA blocker is the top checklist. @matthewdeng to find owner.

[matthewdeng]

@richardliaw quick update on this:

1. I am not planning to remove dead code and tests for 2.7.
2. There is some minor cleanup in flight, e.g. for cleaning up some additional AIR/Train API references
3. I am planning to do another pass over for references to AIR (in particular in the Train docs)

Could you start doing a broader pass over all the documentation, to see if there is anything else?