Add workflow to comment the PR docs

Author: lkdvos

.github/workflows/PR_comment.yml

@@ -0,0 +1,17 @@
+name: Docs preview comment
+on:
+  pull_request:
+    types: [labeled]
+
+permissions:
+  pull-requests: write
+jobs:
+  pr_comment:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Create PR comment
+        if: github.event_name == 'pull_request' && github.repository == github.event.pull_request.head.repo.full_name && github.event.label.name == 'documentation'
+        uses: thollander/actions-comment-pull-request@v3
+        with:
+          message: 'After the build completes, the updated documentation will be available [here](https://quantumkithub.github.io/MatrixAlgebraKit.jl/previews/PR${{ github.event.number }}/)'
+          comment-tag: 'preview-doc'