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 "temporary storage" has a clearer/more specific meaning for you than it does for me. Would be interested to see how others feel about this? (Honestly, we could probably just drop this bullet without losing much.)