Add Product hubs landing page to V2 nav

References docs-content://products/index.md so /products/ resolves to
the landing page instead of 404. The landing lists Elasticsearch and
Kibana hubs alongside stubs for upcoming products and deployment
runbooks. Companion docs-content commit lands the index.md itself on
the hub-pages branch.

Author: florent-leborgne

config/navigation-v2.yml

@@ -3310,6 +3310,8 @@ nav:
   - section: Product hubs
     url: /products/
     children:
+    - page: docs-content://products/index.md
+      title: Product hubs
     - page: docs-content://products/kibana/v9.md
       title: Kibana
     - page: docs-content://products/elasticsearch/v9.md