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.
I'm trying to address different levels of novices and experts with the redundancy here. The earlier bullets are for "I have no idea what I'm doing", and the later bullets are "I know what I want, which is the right implementation of *type of collection*? In this case, HashMap is what you pick when you "just want a map".