docs: key the concurrency group on the ref

A constant `pages` group meant every PR docs build contended for one slot.
GitHub cancels the pending run in a group when a new one queues, so a burst
of Dependabot PRs left all but the last with no `build` check at all -- and
`build` is a required status check, so those PRs sat BLOCKED forever.

Keyed on github.ref, PR builds no longer collide. cancel-in-progress is on
only for pull_request, so a default-branch Pages deployment is never killed
mid-flight.

Author: soodoku

.github/workflows/docs.yml

@@ -21,8 +21,8 @@ permissions:
 
 # Allow only one concurrent deployment, skipping runs queued between the run in-progress and latest queued.
 concurrency:
-  group: "pages"
-  cancel-in-progress: false
+  group: docs-${{ github.ref }}
+  cancel-in-progress: ${{ github.event_name == 'pull_request' }}
 
 jobs:
   # Build documentation