Preview certificate authentication

Author: cherylmc

articles/vpn-gateway/TOC.yml

@@ -93,14 +93,16 @@
         href: vpn-gateway-delete-vnet-gateway-powershell.md
   - name: Site-to-site (S2S)
     items:
-    - name: Configure site-to-site connections
+    - name: Configure site-to-site connections - shared key
       items:
       - name: Azure portal
         href: tutorial-site-to-site-portal.md
       - name: Azure PowerShell
         href: vpn-gateway-create-site-to-site-rm-powershell.md
       - name: Azure CLI
         href: vpn-gateway-howto-site-to-site-resource-manager-cli.md
+    - name: Configure site-to-site connections - certificate
+      href: site-to-site-certificate-authentication-gateway-portal.md
     - name: Add or remove site-to-site-connections
       href: add-remove-site-to-site-connections.md
     - name: Connect to multiple policy-based VPN devices