This PR knocks off a few miscellaneous GA docs P0s given in our docs tracker. Namely:
- Documents Datasets resource allocation model.
- De-emphasizes global/windowed shuffling.
- Documents lazy execution mode, and expands our execution model docs in general.
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.
