add docs for control plane outbound lb

Author: shysank

docs/book/src/SUMMARY.md

@@ -8,6 +8,7 @@
     - [AAD Integration](./topics/aad-integration.md)
     - [API Server Endpoint](./topics/api-server-endpoint.md)
     - [Cloud Provider Config](./topics/cloud-provider-config.md)
+    - [Control Plane Outbound Load Balancer](./topics/control-plane-outbound-lb.md)
     - [Custom Private DNS Zone Name](./topics/custom-dns.md)
     - [Custom Images](./topics/custom-images.md)
     - [Data Disks](./topics/data-disks.md)

docs/book/src/topics/control-plane-outbound-lb.md

@@ -0,0 +1,38 @@
+# Control Plane Outbound Load Balancer
+
+This document describes how to configure your clusters' control plane outbound load balancer.
+
+### Public Clusters
+
+For public clusters ie. clusters with api server load balancer type set to `Public`, CAPZ automatically does not support adding a control plane outbound load balancer.
+This is because the api server load balancer already allows for outbound traffic in public clusters.
+
+### Private Clusters
+
+For private clusters ie. clusters with api server load balancer type set to `Internal`, CAPZ does not create a control plane outbound load balancer by default. 
+To create a control plane outbound load balancer, include the `controlPlaneOutboundLB` section with the desired settings. 
+
+Here is an example of configuring a control plane outbound load balancer with 1 front end ip for a private cluster:
+
+```yaml
+apiVersion: infrastructure.cluster.x-k8s.io/v1alpha4
+kind: AzureCluster
+metadata:
+  name: my-private-cluster
+  namespace: default
+spec:
+  location: eastus
+  networkSpec:
+    apiServerLB:
+      type: Internal
+    controlPlaneOutboundLB:
+      frontendIPsCount: 1
+```
+
+<aside class="note warning">
+
+<h1> Warning </h1>
+
+The field `controlPlaneOutboundLB` cannot be modified after cluster creation. Trying to do so will result in a validation error.
+
+</aside>