Add preview docs action

Author: lkdvos

.github/workflows/PR_comment.yml

@@ -0,0 +1,16 @@
+name: pr_comment
+on:
+  pull_request:
+    types: [opened]
+  paths:
+    - 'docs/**'
+
+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
+        uses: thollander/actions-comment-pull-request@71efef56b184328c7ef1f213577c3a90edaa4aff
+        with:
+          message: 'After the build completes, the updated documentation will be available [here](https://quantumkithub.github.io/PEPSKit.jl/previews/PR${{ github.event.number }}/)'