This PR fixes the issue of diverging documentation between Ray Docs and ecosystem library readmes which live in separate repos (eg. xgboost_ray). This is achieved by adding an extra step before the docs build process starts that downloads the readmes of specified ecosystem libraries from their GitHub repositories. The files are then preprocessed by a very simple parser to allow for differences between GitHub and Docs markdowns.
In summary, this makes the markdown files in ecosystem library repositories single sources of truth and removes the need to manually keep the doc pages up to date, all the while allowing for differences between what's rendered on GitHub and in the Docs.
See ray-project/xgboost_ray#204 & https://ray--23505.org.readthedocs.build/en/23505/ray-more-libs/xgboost-ray.html for an example.
Needs ray-project/xgboost_ray#204 and ray-project/lightgbm_ray#30 to be merged first.
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).