Rollup merge of #151589 - Urgau:documentation-scope, r=GuillaumeGomez

Add a `documentation` remapping path scope for rustdoc usage

This PR adds a new remapping path scope for rustdoc usage: `documentation`, instead of rustdoc abusing the other scopes for it's usage.

Like remapping paths in rustdoc, this scope is unstable. (rustdoc doesn't even have yet an equivalent to [rustc `--remap-path-scope`](https://doc.rust-lang.org/nightly/rustc/remap-source-paths.html#--remap-path-scope)).

I also took the opportunity to add a bit of documentation in rustdoc book.

Author: Zalathar