Test internal docs on CI

[clarfonthey]

I recently submitted #46979 because the rustdoc for rustc failed to compile, because some of the code snippets were invalid syntax.

I think that it would be reasonable to somehow test this via CI to make sure that the docs compile, even if they aren't included in the main distribution of Rust. I also think that doc tests for internal rustc stuff should be tested alongside everything else.

[clarfonthey]

So I just realised that #46278 was opened for this. I'll leave this open until that's merged.