Duplicate for #25247.
Adds a fix for Dask-on-Ray. Previously, for tasks with multiple return values, we implicitly allowed returning a dict with the return index as the key. This was used by Dask-on-Ray, but this is not documented behavior, and we now require task returns to be iterable instead.
Adds support for Python generators instead of just normal return functions when a task has multiple return values. This will allow developers to cut down on total memory usage for tasks, as they can free previous return values before allocating the next one on the heap.
The semantics for num_returns are about the same as usual tasks - the function will throw an error if the number of values returned by the generator does not match the number of return values specified by the user. The one difference is that if num_returns=1, the task will throw the usual Python exception that the generator cannot be pickled.
As an example, this feature will allow us to reduce memory usage in Datasets shuffle operations (see #25200 for a prototype).
This PR makes a number of major overhauls to the Ray core docs:
Add a key-concepts section for {Tasks, Actors, Objects, Placement Groups, Env Deps}.
Re-org the user guide to align with key concepts.
Rewrite the walkthrough to link to mini-walkthroughs in the key concept sections.
Minor tweaks and additional transition material.
