Add GitHub Actions workflow to trigger site rebuild

.github/workflows/trigger-site-build.yml

@@ -0,0 +1,18 @@
+name: Trigger site rebuild
+
+on:
+  push:
+    branches: [master]
+    paths:
+      - 'docs/**'
+
+jobs:
+  trigger:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Trigger GitLab pipeline
+        run: |
+          curl -sS -X POST \
+            -F "token=${{ secrets.GITLAB_TRIGGER_TOKEN }}" \
+            -F "ref=master" \
+            "https://gitlab.com/api/v4/projects/83371847/trigger/pipeline"