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.
The first bullet, about temporary storage -- I'm curious what the justification is? I don't see how it argues for `Vec` over other collections, personally.