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.
Ok, so the real meaning is: "You want a no-frills map" :-)

This feels a bit similar to the issue with the bullet for `Vec` above. Maybe you can say: "You only need basic map operations"? Or something.