[docs] Revamp README and Ray intro doc page (ray-project#27405)

This PR revamps and aligns the README and Ray intro doc page:

New "What is Ray" diagram that introduces AIR vs Ray core (diagram TBD finalized, this is the working placeholder)
Update the description of Ray
Link out to the user guides for key libraries and key concepts
Remove old / broken links, as well as the inline library descriptions from the README

.github/CODEOWNERS

@@ -8,7 +8,7 @@
 # ==== Documentation ====
 
 # Authors responsible for copy-editing of the documentation.
-/doc/ @maxpumperla @pcmoritz @ericl @stephanie-wang @dmatrix @sriram-anyscale
+/doc/ @maxpumperla @pcmoritz @ericl @stephanie-wang @dmatrix @sriram-anyscale @richardliaw
 
 # ==== Ray core ====