Make duplication instructions more visible

Rule duplication instructions are Portal-specific, but should be searchable regardless of interface used. Making it its own separate sub-section under "Create rules"

Author: andreamichaelmsft

articles/virtual-network/manage-network-security-group.md

@@ -386,6 +386,7 @@ az network nsg rule create \
     --protocol Tcp \
     --description "Allow RDP"
 ```
+---
 
 #### Duplicate security rules