Update some doc links.

Update some doc links.

This updates some links to their new locations.

This also switches to relative-style cross-book links. This allows the links
to work better when viewing local documentation (such as through rustup),
keeps links to the same toolchain (not just stable), and allows the rust-repo
linkchecker to validate the links. The downside is that when working on the
documentation in the cargo repo, the links won't work.