Add an overview toggle to rustdoc lib output

[mdinger]

Rustdoc libs should have an overview button of some sort.

As discussed here, one problem with the std docs is that you can't tell which part of an unfamiliar page is actually useful. For example, below is a mockup of what the main regex page would look like with and without the types which can only be created indirectly. Clearly the top part is much more approachable to someone unfamiliar with this library.

This seems a somewhat reasonable thing to do although I'm not certain expand/overview are the best words or icons for this but it seems okay I guess. One alternative would be to only show indirectly created types where they're being returned (but that is more drastic and it seems it could be done after this was tried if this was really well liked).

cc @BurntSushi since this interested you

[steveklabnik]

triage: no change

[steveklabnik]

Triage: no change

[RodBurman]

The documentation does look different and label names have been changed. Documentation is never going to be perfect for everyone, but given there has been no comment on this for 5 years lets close this issue and await a new set of constructive criticism of the current state.