This adds a multinode checkpoint/restore test for Ray Tune. This covers some of the functionality of the release tests, but in a more controlled environment. In a follow-up PR, we should test (mocked) cloud checkpointing, too.
The current multi node tests use a hardcoded mapping for local development mounts. With this PR, a new environment variable is introduced to be able to control this dynamically. Additionally, some minor improvements to the test utilities and monitor are added.
This PR consolidates both #21667 and #21759 (look there for features), but improves on them in the following way:
- [x] we reverted renaming of existing projects `tune`, `rllib`, `train`, `cluster`, `serve`, `raysgd` and `data` so that links won't break. I think my consolidation efforts with the `ray-` prefix were a little overeager in that regard. It's better like this. Only the creation of `ray-core` was a necessity, and some files moved into the `rllib` folder, so that should be relatively benign.
- [x] Additionally, we added Algolia `docsearch`, screenshot below. This is _much_ better than our current search. Caveat: there's a sphinx dependency that needs to be replaced (`sphinx-tabs`) by another, newer one (`sphinx-panels`), as the former prevents loading of the `algolia.js` library. Will follow-up in the next PR (hoping this one doesn't get re-re-re-re-reverted).
