Update the ARO doc about the CIDR using in OVN

We have customers having issue when they deploy an ARO 4.11, and after troubleshooting we believe the issue is that OVN is using CIDR 100.64.0.0/16 internally so they must not have this IP range in their ARO subnets.  However, customer is hard to find this information from our existing Azure OpenShift Docs.  

We have the part of information included in the Red Hat OpenShift documentation ((https://access.redhat.com/documentation/en-us/openshift_container_platform/4.11/html/networking/ovn-kubernetes-default-cni-network-provider) but not included in the ARO doc. As requested by Red Hat CEE and MSFT, we create this PR and update in this ARO doc to reflect this issue accordingly.

Author: xiaoyu74

articles/openshift/concepts-ovn-kubernetes.md

@@ -27,6 +27,8 @@ The OVN-Kubernetes CNI cluster network provider offers the following features:
 
 > [!NOTE]
 > As of ARO 4.11, OVN-Kubernetes is the CNI for all ARO clusters. In already existing clusters, migrating from the previous SDN standard to OVN is not supported.
+> 
+> If your cluster uses any part of the 100.64.0.0/16 IP address range, you cannot migrate to OVN-Kubernetes because it uses this IP address range internally.
 
 For more information about OVN-Kubernetes CNI network provider, see [About the OVN-Kubernetes default Container Network Interface (CNI) network provider](https://docs.openshift.com/container-platform/latest/networking/ovn_kubernetes_network_provider/about-ovn-kubernetes.html).