ci: add workflow to notify main repo on docs update

Triggers NexusGate main repo's update-docs-submodule workflow
when docs are pushed to main branch.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: pescn

.github/workflows/notify-main-repo.yml

@@ -0,0 +1,17 @@
+name: Notify Main Repo
+
+on:
+  push:
+    branches: [main]
+
+jobs:
+  notify:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Trigger main repo workflow
+        uses: peter-evans/repository-dispatch@v3
+        with:
+          token: ${{ secrets.MAIN_REPO_PAT }}
+          repository: EM-GeekLab/NexusGate
+          event-type: docs-updated
+          client-payload: '{"message": "Docs updated: ${{ github.event.head_commit.message }}"}'