Add user guide for ServerTlsPolicy (#15346)

Author: zli82016

mmv1/products/networksecurity/ServerTlsPolicy.yaml

@@ -17,6 +17,7 @@ description: |
   ServerTlsPolicy is a resource that specifies how a server should authenticate incoming requests. This resource itself does not affect configuration unless it is attached to a target HTTPS proxy or endpoint config selector resource.
 references:
   guides:
+    'Use ServerTlsPolicy': 'https://cloud.google.com/load-balancing/docs/mtls'
   api: 'https://cloud.google.com/traffic-director/docs/reference/network-security/rest/v1beta1/projects.locations.serverTlsPolicies'
 docs:
 base_url: 'projects/{{project}}/locations/{{location}}/serverTlsPolicies'