Add workflow to trigger index repo

.github/workflows/webhook.yaml

@@ -0,0 +1,12 @@
+name: Push changes to upstream subtree
+on: [push]
+
+jobs:
+  trigger-index-workflow:
+    runs-on: ubuntu-latest
+    steps:
+      - run: |
+        curl -x POST -H "Authorization: token ${GH_WORKFLOW_PAT}" -H "Content-Type: application/json" \
+          https://api.github.com/repos/infogulch/index/dispatches \
+          -d '{"event_type": "notebooks_push"}'
+