updated TOC

Author: MGoedtel

articles/azure-arc/toc.yml

@@ -14,6 +14,10 @@
       href: ./servers/onboard-service-principal.md
     - name: Connect machines to Azure Arc with PowerShell DSC
       href: ./servers/onboard-dsc.md
+  - name: Manage
+    items:
+    - name: Manage agent
+      href: ./servers/manage-agent.md
 - name: Reference
   items:
     - name: Azure Connected Machine Agent CLI interface