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 think, before this section, it might be good to have a very concise section "What collections are available?" that simply lists each collection type and says what it is in somewhat technical terms. E.g., `HashMap` could say "A hash map based on the Robin Hood hashing strategy" or some such.