Add generated docs for crate_universe

[UebelAndre]

Additionally, this renames crate.override.extra_rust_env_vars to crate.override.extra_rustc_env_vars to be more consistent with the behavior of that field

[UebelAndre]

These docs aren't surfaced in the top level page, but are included in flatten.md. I figured since the rules were still experimental that we might not want to advertise them yet? I'm happy to add that though.

[illicitonion]

Thanks!