chore: support merge queues in CI

Add the merge_group event to the CI workflows so the checks also run against
the merge commit built by the merge queue. Without it the required checks
never report on a queued pull request and the queue stalls.

Author: NSeydoux

.github/workflows/site-ci-config.yml

@@ -5,6 +5,8 @@ on:
   pull_request:
     paths:
       - '**/site/**'
+  # Run the required tests on the merge queue.
+  merge_group:
 
 permissions: {}