Link to `hir_attrs` in `/hir/attribute-parsing.md` leads to Not Found

[josetorrs]

I was reading through the dev-guide while working on an issue and the link to hir_attrs (https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/attrs/index.html) leads a Not Found site. I'm able to look through https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/index.html but I'm not sure if that's the right link.

Related issue: #1896

[mejrs]

I split up attributes away from rustc_hir, thats why some links are broken now.