Add refs to network plugins

Author: jab-rh

networking/about-networking.adoc

@@ -11,8 +11,8 @@ toc::[]
 The following list highlights some of the most commonly used {openshift-networking} features available on your cluster:
 
 - Primary cluster network provided by either of the following Container Network Interface (CNI) plugins:
-  * OVN-Kubernetes network plugin, the default
-  * OpenShift SDN network plugin, an optional alternative
+  * xref:../networking/ovn_kubernetes_network_provider/about-ovn-kubernetes.adoc#about-ovn-kubernetes[OVN-Kubernetes network plugin], the default plugin
+  * xref:../networking/openshift_sdn/about-openshift-sdn.adoc#about-openshift-sdn[OpenShift SDN network plugin]
 - Certified 3rd-party alternative primary network plugins
 - Cluster Network Operator for network plugin management
 - Ingress Operator for TLS encrypted web traffic