Add -Z normalize-docs and enable it for compiler docs

[jyn514]

Works around #79459 by only enabling normalization for the compiler itself (and anyone who opts-in on nightly). Eventually I want to remove this and enable normalization by default, but that's turned out to be really hard.

This uses a command line option instead of a feature gate so it's easier to pass it to all crates at once. Theoretically it's better to use a feature gate instead so that it's easier for people to use on docs.rs, but I'm also not terribly concerned with how easy it to use a temporary hack.

Addresses #77459.

[rust-highfive]

r? @GuillaumeGomez

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[jyn514]

(I'm testing locally that this actually works and generates the correct docs, this probably shouldn't be approved until then.)

[jyn514]

Update: it does not work, I think I modified the wrong part of bootstrap.

[jyn514]

Ok, it's working now. Not sure if I should add a test that the option is unstable - it uses -Z so I'd assume it's just as unstable as every other option?

[GuillaumeGomez]

If you have to use -Z unstable, then it's unstable. Also, I'd like to have someone from the @rust-lang/compiler tool to check this PR as well to be sure we didn't forget anything.

[nagisa]

None of the -Z options actually require -Zunstable-options. -Z by itself is intended to signal the instability and stable compiler will blanket-reject any and all -Z options. The -Zunstable-options flag exists to enable making non--Z flags (such as --pretty) unstable.

As long as the newly added flag is in the DebuggingOptions section you're good on stability.

[jyn514]

@bors r=GuillaumeGomez

We'll finally have compiler docs that look nice 🎉

[bors]

📌 Commit 95a6427 has been approved by GuillaumeGomez