Adds a from_huggingface method to Datasets, which allows the conversion of a Hugging Face Dataset to a Ray Dataset. As a Hugging Face Dataset is backed by an Arrow table, the conversion is trivial.
1. Dataset pipeline is advanced usage of Ray Dataset, which should not jam into the Getting Started page
2. We already have a separate/dedicated page called Pipelining Compute to cover the same content
This PR adds experimental support for random access to datasets. A Dataset can be random access enabled by calling `ds.to_random_access_dataset(key, num_workers=N)`. This creates a RandomAccessDataset.
RandomAccessDataset partitions the dataset across the cluster by the given sort key, providing efficient random access to records via binary search. A number of worker actors are created, each of which has zero-copy access to the underlying sorted data blocks of the Dataset.
Performance-wise, you can expect each worker to provide ~3000 records / second via ``get_async()``, and ~10000 records / second via ``multiget()``.
Since Ray actor calls go direct from worker->worker, throughput scales linearly with the number of workers.
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.
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.
