Instead of relying on the node-ip custom resource for static task-to-node placement, this PR introduces an explicit NodeAffinitySchedulingStrategy with the following benefits:
1. Specify node using id instead of ip since ip may not be unique for each node.
2. Support soft constraint so the task can be tolerant to node failures.
After this PR, the node-ip custom resource can be deprecated.
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.
