library-level docs for collections
add missing btree re-exports
auto merge of #17802 : Gankro/rust/collection-docs-redux, r=aturon

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.