include librustdoc in the distributed compiler docs

[QuietMisdreavus]

#49193 created a bootstrap dist task to create docs for the rustc crates, but this does not include the rustdoc crate.

From a discussion on IRC just now:

<davidtwco> misdreavus, SimonSapin: The rustc doc step runs cargo doc with explicit args for the crates to include. It has a blacklist ignoring some root level crates and then does a DFS on cargo metadata to get the children of that.
<davidtwco> So if librustdoc is not a child of the top most rustc crate. It won't be included.
<davidtwco> You can add a new doc step that runs in the librustdoc folder (more or less the same code as the rustc step) and output it to the same compiler docs folder. Then it'll get picked up by the dist step.