Make links from wgpu-types to wgpu work in docs.rs.

[kpreid]

As a bonus, the macro now allows defining such links while writing the item name only once instead of twice.

Connections
Fixes #8199 (mostly).

Description
../<other crate> is not sufficient to link to another crate on docs.rs. This PR adds a macro which generates links that will work on docs.rs when cfg(docsrs) is set. The macro only applies to wgpu-types; it would be trickier to make it work for links in wgpu-core and wgpu-hal, but those links were fewer and less significant, so I figured it wasn't immediately worth doing.

I discovered a presumed bug in docs.rs, rust-lang/docs.rs#3036, and this PR contains a workaround for it which will link to older documentation than ideal. The alternative would be to embed the latest version number of wgpu into wgpu-types, which seems worse.

Testing

• Clicked links in cargo doc to test local docs
• Ran RUSTDOCFLAGS='--cfg=docsrs' cargo doc, manually edited together the resulting URLs with https://docs.rs/, and verified they worked in docs.rs.

Squash or Rebase?
One commit

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy --tests. If applicable, add:
  • --target wasm32-unknown-unknown
• Run cargo xtask test to run tests.
• If this contains user-facing changes, add a CHANGELOG.md entry.