add std and core to nightly-rustc docs

[lolbinarycat]

the rustc internal api docs differ from the main std docs in two major ways:

1. documenting different crates
2. showing private items

being able to see std-internal items would be quite handy for anyone developing the standard library.

[ChrisDenton]

In the past there's been concern about it showing up in search results and people getting confused. That's one reason why the third party stdrs.dev has a big red warning sign.

[lolbinarycat]

there should probably be some sort of heading on the nightly-rustc docs, yeah.

the fact that there's no heading means i kinda just have to refer to them by the url.

generally speaking, documentation doesn't link to other docs enough.

[jieyouxu]

I actually do like the current setup where nightly rustc crates + tool crates + bootstrap docs get their own search subsite that is independent from std docs. It's already quite difficult at times to find something inside compiler or tools through search, I wouldn't want those search results to also include std things. That is, I actually prefer the current setup where:

• Rustc, tools and bootstrap get their nightly-rustc subsite https://doc.rust-lang.org/nightly/nightly-rustc/ which includes --document-private-items, and
• Std get its own nightly subsite like <stdrs.dev> (though we can "officially" build those too FWIW).

Unless there's already an easy way in rustdoc to restrict where results from come.

EDIT: I would be perfectly fine with building them together if we have multi-crate search result filters #130971.