OSDOCS-1855: OVN Egress Router

Replace old doc with the new process that uses
the `EgressRouter` object.

Feedback from Daniel:
* Acknowledge that the Operator adds the NAD and deployment.
* The operator also removes them when an egress router is
  deleted.
* Change "Failover config" to describe the deployment and
  mention svc rather that suggest customers can do this for
  themselves.

Remove the technology preview admonition.

Remove file modules/nw-egress-router-network-attachment.adoc because
customers do not need to make the NAD themselves after this release.

Change from "Partial support" to "Supported" in the
Supported default CNI network provider feature matrix and change
the numbering to be *more* sequential.

Add `fallbackIP` based on openshift/cluster-network-operator#1094.

Specify IP address in CIDR notation. Correction from Mohamed. Remove
the `spec.networkInterface` field, also from Mohamed.

Update migration process to indicate redirect is supported and
that HTTP mode and DNS mode are not supported. Also indicate
that the Operator does not offer a provision for a node selector.

Add `metadata.namespace` to the YAML.

Review from Ashley.

Author: Mike McKiernan

modules/nw-ovn-kubernetes-migration-about.adoc

@@ -1,3 +1,21 @@
+// Module included in the following assemblies:
+//
+// * networking/ovn_kubernetes_network_provider/migrate-from-openshift-sdn.adoc
+
+[id="nw-ovn-kubernetes-migration-about_{context}"]
+= Migration to the OVN-Kubernetes network provider
+
+Migrating to the OVN-Kubernetes Container Network Interface (CNI) cluster network provider is a manual process that includes some downtime during which your cluster is unreachable. Although a rollback procedure is provided, the migration is intended to be a one-way process.
+
+[id="considerations-migrating-ovn-kubernetes-network-provider_{context}"]
+== Considerations for migrating to the OVN-Kubernetes network provider
+
+The subnets assigned to nodes and the IP addresses assigned to individual pods are not preserved during the migration.
+
+While the OVN-Kubernetes network provider implements many of the capabilities present in the OpenShift SDN network provider, the configuration is not the same.
+
+* If your cluster uses any of the following OpenShift SDN capabilities, you must manually configure the same capability in OVN-Kubernetes:
++
 --
 * Namespace isolation
 * Egress IP addresses