Add the `#[diagnostic]` attribute namespace and the `#[diagnostic::on_unimplemented]` feature to the reference

[weiznich]

As requested here

[weiznich]

I'm not really sure what needs to be done to address the CI errors as the name space is in fact currently unstable but it was requested during the stablization PR to fill this PR.

[ehuss]

The CI errors are fine, this won't be merged until the stabilization hits nightly which should resolve them.

[compiler-errors]

This can/should get merged now