This PR allows the user to override the global default for max_retries for non-actor tasks. It adds an OS env called RAY_task_max_retries which can be passed to the driver or set with runtime envs. Any future tasks submitted by that worker will default to this value instead of 3, the hard-coded default.
It would be nicer if we could have a standard way of setting these defaults, but I think this is fine as a one-off for now (not a clear need for overriding defaults of other @ray.remote options yet).
Related issue number
Closes#24854.
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.
