Document builtin attributes (like `no_mangle`, `repr`, ...) in the standard library docs

[Veykril]

Location

https://doc.rust-lang.org/std/

Summary

The standard library docs currently document keywords on the main index https://doc.rust-lang.org/std/#keywords

I propose we should do the same for builtin language attributes (no_mangle, repr, no_std), but notably not builtin attribute macros (test, bench`) as those are already described by nature of being proper macros exposed from the standard libraries.

They should be using a similar mechanism as to how keywords are documented, dummy modules with doc strings. That way tooling like IDEs can render these docs on hover of the builtin language attributes.

cc @GuillaumeGomez