Add reusable workflow

Author: reakaleek

.github/workflows/comment-on-asciidoc-changes.yml

@@ -0,0 +1,35 @@
+name: Comment on PR for .asciidoc changes
+
+on:
+  workflow_call: ~
+
+jobs:
+  comment-on-asciidoc-change:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout the repository
+        uses: actions/checkout@v3
+        with:
+          fetch-depth: 0  # This is important to fetch all history
+
+      - name: Check for changes in .asciidoc files
+        id: check-files
+        run: |
+          git fetch origin ${{ github.base_ref }}
+          if git diff --name-only origin/${{ github.base_ref }}..HEAD | grep -E '\.asciidoc$'; then
+            echo "asciidoc_changed=true" >> $GITHUB_OUTPUT
+          else
+            echo "asciidoc_changed=false" >> $GITHUB_OUTPUT
+          fi
+      - name: Add a comment if .asciidoc files changed
+        if: steps.check-files.outputs.asciidoc_changed == 'true'
+        uses: actions/github-script@v6
+        with:
+          script: |
+            github.rest.issues.createComment({
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              issue_number: context.payload.pull_request.number,
+              body: 'It looks like this PR modifies one or more `.asciidoc` files. The documentation is currently under a documentation freeze. Please do not merge this PR. See the [migration guide](https://elastic.github.io/docs-builder/migration/index.html) to learn more.'
+            })