rustdoc: Create anchor pages for primitive types

This is currently rebased on top of #14478, but that's just to preemptively avoid rebase conflicts and to provide a better preview. This can land independently of that PR.

This change crates a dedicated page in rustdoc for primitive types to outline everything you can do with them (at least in a basic way).

* Preview - http://people.mozilla.org/~acrichton/doc/
* Exhibit A - http://people.mozilla.org/~acrichton/doc/std/#primitives
* Exhibit B - http://people.mozilla.org/~acrichton/doc/std/primitive.str.html
* Exhibit C - http://people.mozilla.org/~acrichton/doc/std/primitive.slice.html

Please don't hesitate to be nitpickity, it's easy to overlook a thing here or there!