Add empty PR docs workflow. (#4879)

.github/workflows/docs-pr.yml

@@ -0,0 +1,9 @@
+name: Collection Docs
+concurrency:
+  group: docs-${{ github.head_ref }}
+  cancel-in-progress: true
+on:
+  pull_request_target:
+    types: [opened, synchronize, reopened, closed]
+
+jobs: {}