Migrate mdbook-spec to this repo.

[ehuss]

This migrates the mdbook-spec plugin from the https://github.com/rust-lang/spec/ repository to this repository. This is done for a few reasons:

• No need to separately install the plugin.
• Makes it easier to work on the plugin. You can just edit the code, and immediately see the results.
• Makes it easier to integrate with rust-lang/rust. No need to separately publish the plugin.

This was made possible by the recent change of https://github.com/rust-lang/rust/pull/127786.

There shouldn't be any changes to the plugin compared to the spec repo.

This changes CI to split the work across multiple jobs. This can make it easier to see which parts are failing, for example if multiple things are failing, before you could only see one at a time. However, this requires a change to the team repo at https://github.com/rust-lang/team/pull/1502 before it can merge.