library-level docs for collections

Adds a high-level discussion of "what collection should you use for what", as well as some general discussion of correct/efficient usage of the capacity, iterator, and entry APIs.

Still building docs to confirm this renders right and the examples are good, but the content can be reviewed now.
Yeah it's a bit borderline, but it seemed... symmetric? My general pattern here is a gradient from "descriptive for newbies" to "what experts know", where "what experts know" is generally generic types like "it's the map" or "it's the set".