Add refstring entry

Author: Pablo Gonzalez

articles/frontdoor/TOC.yml

@@ -384,6 +384,8 @@
         href: ../web-application-firewall/afds/waf-front-door-configure-ip-restriction.md?toc=/azure/frontdoor/toc.json
     - name: Troubleshoot
       items:
+      - name: Troubleshooting with RefString
+        href: refstring.md
       - name: Troubleshooting Front Door issues
         href: troubleshoot-issues.md
       - name: Troubleshooting Front Door performance issues