Preview: [docs](https://ray--21931.org.readthedocs.build/en/21931/data/dataset.html)
The Ray Data project's docs now have a clearer structure and have partly been rewritten/modified. In particular we have
- [x] A Getting Started Guide
- [x] An explicit User / How-To Guide
- [x] A dedicated Key Concepts page
- [x] A consistent naming convention in `Ray Data` whenever is is referred to the project.
This surfaces quite clearly that, apart from the "Getting Started" sections, we really only have one real example. Once we have more, we can create an "Example" section like many other sub-projects have. This will be addressed in https://github.com/ray-project/ray/issues/21838.
This PR finishes most of the stats todos for dataset. The main thing punted for future work is instrumentation of split(), which is particularly tricky since only certain blocks are transformed.
Co-authored-by: Clark Zinzow <clarkzinzow@gmail.com>
Datasets docs for last-mile preprocessing, particularly geared towards ML ingest. This gives groupby, aggregations, and random shuffling examples in the overview page (not present previously), adds some concreteness to our last-mile preprocessing positioning, and provides some preprocessing recipes for a few common transformations.
