Add migration guide for NAT Gateway to StandardV2

Author: aimee-littleton

articles/nat-gateway/toc.yml

@@ -13,6 +13,8 @@ items:
         href: nat-gateway-resource.md
       - name: NAT Gateway SKUs
         href: nat-sku.md
+      - name: Migrate NAT Gateway to StandardV2
+        href: nat-gateway-v2-migrate.md
   - name: Reliability
     items:
       - name: Reliability in Azure NAT Gateway