Update TOC with Security node, add new article

Author: jpconnock

articles/azure-app-configuration/TOC.yml

@@ -64,8 +64,6 @@
   items:
     - name: Key-value store
       href: concept-key-value.md
-    - name: Encrypt using customer-managed keys
-      href: concept-customer-managed-keys.md
     - name: Point-in-time snapshot
       href: concept-point-time-snapshot.md
     - name: Feature management
@@ -76,8 +74,10 @@
       href: concept-app-configuration-event.md
     - name: Secure your config store using Private Endpoints
       href: concept-private-endpoint.md
-    - name: Authentication
+    - name: Security
       items:
+        - name: Encrypt using customer-managed keys
+          href: concept-customer-managed-keys.md
         - name: Integrate with Azure Managed Identity
           href: howto-integrate-azure-managed-service-identity.md
         - name: Enable access using Azure Active Directory