Move primitive docs to `core`

[camelid]

Rust primitives are currently documented in std, but in some cases we want to link to primitives from core (cc #83577). Also, core seems like it would be a better place to document primitives since they are available in programs that are no_std (yes, they're also available in no_core programs, but we have to put the docs somewhere).

We would probably have to re-export the primitives in std (and alloc too?) so as not to break links.

What do you (T-rustdoc, T-libs) think?

cc @rust-lang/rustdoc

[jyn514]

This has been discussed before in #73423, I would rather keep discussion in one issue.

[camelid]

Sounds good, I didn't know if there was a previous issue.