Add alerting-rule-templates to the nav tree

Author: alaudazzi

reference/fleet/toc.yml

@@ -150,6 +150,7 @@ toc:
           - file: data-streams-pipeline-tutorial.md
           - file: data-streams-advanced-features.md
       - file: alert-templates.md
+      - file: alerting-rule-templates.md
   - file: agent-command-reference.md
   - file: providers.md
     children: