Stabilize intra-doc links
Fixes https://github.com/rust-lang/rust/issues/43466
Thanks to the great work of `@jyn514` in getting the [cross-crate reexport issue](https://github.com/rust-lang/rust/issues/65983) in intra-rustdoc links fixed, I think we're now in a position to stabilize this feature.
The tracking issue currently has two unresolved issues:
- <s>behavior around doc(hidden): This is fixed in https://github.com/rust-lang/rust/pull/73365, which is just waiting for CI and should land tomorrow. It's also a pretty niche bug so while I expect it to land soon I don't think we need to block stabilization on it anyway.</s>
- Non-identifier primitive types like slices: This was not a part of the original RFC anyway, and is a pretty niche use case
The feature itself, sans https://github.com/rust-lang/rust/issues/65983, has been shipped on nightly for three years now, with people using it on docs.rs. https://github.com/rust-lang/rust/issues/65983 itself is not an overwhelmingly central bit of functionality; the reason we elected to block stabilization on it was that back in 2017 it was not possible to fix the issue without some major refactorings of resolve, and we did not want to stabilize something that had such a potentially unfixable bug.
Given that we've fixed it, I see no reason to delay stabilization on this long awaited feature. It's possible that the latest patches have problems, however we _have_ done crater runs of some of the crucial parts. Furthermore, that's what the release trains are for, we will have a solid three months to let it ride the trains before it actually hits the stable compiler.
r? `@rust-lang/rustdoc`
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