Support feature-gated tests during PR review

[ehuss]

@traviscross and I were discussing how to improve the workflow with PRs that have examples for new features that are being stabilized, but have not yet stabilized in nightly.

The problem is that the CI checks forbid #![feature(...)], because we want to ensure that the reference only documents stable features. But without #![feature()] the examples will fail because it is missing.

One idea we had to partially smooth over this is to allow #![feature(...)] in the pull_request CI trigger, but emit a warning which should show up fairly clearly in the "files" view. Then, in the merge_group CI trigger, it can make that an error to ensure we don't accidentally merge it.

So the workflow would be roughly:

1. Open reference PR with #![feature()] as needed in examples
2. After review approval, wait for the stabilization to hit nightly
3. Remove #![feature()] from the reference PR
4. Merge the reference PR