Rollup merge of #128215 - ehuss:update-reference, r=Kobzol

Update the reference

This updates the reference to use the new mdbook-spec preprocessor, which is a Cargo library inside the reference submodule.

Note that this PR contains a bunch of bootstrap cleanup commits to assist with making sure the submodules are working correctly. All of the cleanup PRs should have a description in their commit. I'd be happy to move those to a separate PR if that makes review easier.

The main changes for the reference are:
- Move the `doc::Reference` bootstrap step out of the generic macro into a custom step.
    - This step needs to build rustdoc because the new mdbook-spec plugin uses rustdoc for generating links.
    - PATH is updated so that the rustdoc binary can be found.
- rustbook now includes the mdbook-spec plugin as a dependency.
- rustbook enables the mdbook-spec preprocessor.

I did a bunch of testing with the various commands and setups, such as:
- `submodules=true` and `submodules=false`
- having all submodules deinitialized
- not in a git repository

However, there are probably thousands of different permutations of different commands, settings, and environments, so there is a chance I'm missing something.

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