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 don't think wants to be fenced *and* indented. One or the other should be enough.

Also, you don't need to import `Vec`.