CI: Skip newsfragment check when label is set (#63983)

* CI: Skip newsfragment check when `skip newsfragment check` label is set

When the `skip newsfragment check` label is applied to a PR, the
newsfragment PR number check workflow is skipped entirely. The workflow
now also triggers on `labeled`/`unlabeled` events so it re-evaluates
when labels change.

* Add skip instructions to newsfragment check error and docs

Update the CI error message to inform users they can add the
'skip newsfragment check' label to bypass the check. Also document
this option in the contribution workflow guide.