[rustdoc] Add new --book-location option to add a link to associated guide and generate it if local

[GuillaumeGomez]

This option adds the possibility to provide a path or a URL to the associated guide of this crate in the crate-level doc page. If the provided argument is a path, then the guide is generated using the mdbook crate.

It looks like this:

You can give it a try here.

It'll need an FCP before getting merged.

Also I need to add tests when the argument is a path but I'll wait for the feature form to be final before doing it to prevent doing it more than once. 😄

cc @notriddle
r? @camelid

[rustbot]

These commits modify the Cargo.lock file. Unintentional changes to Cargo.lock can be introduced when switching branches and rebasing PRs.

If this was unintentional then you should revert the changes before this PR is merged.
Otherwise, you can ignore this comment.

Some changes occurred in HTML/CSS/JS.

cc @GuillaumeGomez, @jsha

[rustbot]

The list of allowed third-party dependencies may have been modified! You must ensure that any new dependencies have compatible licenses before merging.

cc @davidtwco, @wesleywiser

[rustbot]

This PR modifies run-make tests.

cc @jieyouxu

[GuillaumeGomez]

Finally fixed all tidy errors. :')