Add RTD link preview

.github/workflows/rtd-link-preview.yml

+name: Read the Docs Pull Request Preview
+on:
+  pull_request_target:
+    types:
+      - opened
+
+permissions:
+  pull-requests: write
+
+jobs:
+  documentation-links:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: readthedocs/actions/preview@v1
+        with:
+          project-slug: "pytensor"