rustdoc: Ensure that all files have been written on windows before exiting

[GuillaumeGomez]

Fixes #109060.

I originally wanted to use rayon::in_scope to ensure that all threads would be waited once we're out, however it adds to many restrictions and requires to keep a &rayon::Scope around, making it quite tricky to use unfortunately.

So what remains is the classical ways to sync with threads. With this implementation though, if a thread crashes for whatever reason, we simply won't know and rustdoc will wait forever, which is pretty bad.

So if anyone has suggestions for improving this, it'd be very welcome.

In the meantime:

r? @ehuss

[GuillaumeGomez]

Just discovered thread_pool, I'll open another PR. Closing this one.