Actions: attempt to validate PR description tests (#2810)

GitHub includes the description comment when it formulates the commit
message, which we don't want as it's pretty messy. We will ask users to
remove the comment but we need to validate it before allowing merging.

Stolen from
https://github.com/orgs/community/discussions/84771#discussioncomment-8039595

Author: phillco

.github/workflows/pr-description.yaml

@@ -0,0 +1,28 @@
+name: PR Description
+
+on:
+  pull_request:
+    types: [opened, edited, reopened]
+
+jobs:
+  validate-pr-description:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Set up workspace
+        uses: actions/checkout@v2
+
+      - name: Validate description
+        run: |
+          # Fetch PR description from env with jq
+          PR_DESCRIPTION=$(jq -r ".pull_request.body" "$GITHUB_EVENT_PATH")
+          KEYWORD="REQUIRED_KEYWORD"
+
+          # Ensure PR author removed the welcome comment
+          if [[ $PR_DESCRIPTION = *"<!--"* ]] || [[ $PR_DESCRIPTION = *"-->"* ]]; then
+            echo "FAILED: Please remove the welcome comment from your PR description."
+            exit 1
+          else
+            echo "OK: Welcome comment is removed your PR description."
+          fi
+
+          echo "PASS: All checks OK!"