add readthedocs preview to pull requests

.github/workflows/documentation-links.yml

@@ -0,0 +1,20 @@
+name: readthedocs preview
+on:
+  pull_request_target:
+    types:
+      - opened
+    # Execute this action only on PRs that touch
+    # documentation files.
+    # paths:
+    #   - "docs/**"
+
+permissions:
+  pull-requests: write
+
+jobs:
+  documentation-links:
+    runs-on: ubuntu-22.04
+    steps:
+      - uses: readthedocs/actions/preview@v1
+        with:
+          project-slug: "gridtk"