doc: Link to the VM routed nic how-to from the device reference.

Signed-off-by: Mark Laing <mark.laing@canonical.com>

doc/reference/devices_nic.md

@@ -381,7 +381,7 @@ IP addresses, gateways and routes
       169.254.0.1
       fe80::1
 
-  For VMs, the gateways must be configured manually or via a mechanism like `cloud-init`.
+  For VMs, the gateways must be configured manually or via a mechanism like `cloud-init` (see the [how to guide](../howto/instances_routed_nic_vm.md)).
 
   ```{note}
   If your container image is configured to perform DHCP on the interface, it will likely remove the automatically added configuration.