I am surprised by the fact that `GetTimeoutError` is not a subclass of `TimeoutError`, which is counter-intuitive and may discourage users from trying the timeout feature in `ray.get`, because you have to "guess" the correct error type. For most people, I believe the first error type in their mind would be `TimeoutError`.
This PR fixes this.
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.
