moved known issue to troubleshooting section

Author: duongau

articles/firewall/toc.yml

@@ -13,8 +13,6 @@ items:
     href: firewall-preview.md
   - name: FAQ
     href: firewall-faq.yml
-  - name: Known issues and limitations
-    href: firewall-known-issues.md
 - name: Design & architecture
   items:
   - name: Policy 
@@ -212,6 +210,10 @@ items:
     href: ../azure-resource-manager/management/relocation/relocation-firewall.md?toc=/azure/firewall/toc.json&bc=/azure/firewall/breadcrumb/toc.json
   - name: Remote work support
     href: remote-work-support.md
+- name: Troubleshooting
+  items:
+  - name: Known issues and limitations
+    href: firewall-known-issues.md
 - name: Reference
   items:
   - name: Azure CLI