[train] minor documentation improvements

[matthewdeng]

Why are these changes needed?

1. Add images to More Frameworks.
2. Remove Transformers section from More Frameworks since there is now a dedicated page.
3. Cleanup API Reference page.
   1. Reorder Train Loop APIs in general usage.
   2. Remove Restore API references since they're already part of the Trainer APIs
   3. Remove BaseTrainer methods because they're already in BaseTrainer
   4. Add additional APIs like Result and FailureConfig.
      1. Note: This makes the AIR APIs fail to generate, but I think this is okay given that we are moving all code to point to the train counterparts, and will be removing the air API references in 2.7.
4. Update ToC.
   1. Remove "Ray Train" prefix for some pages.
   2. Reorder pages so Examples come before Internals.
   3. Remove unneeded dropdown icon for APIs.

Related issue number

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 :(

[woshiyyya]

Thanks @matthewdeng ! Looks good to me. Left 2 minor comments.