Improve the documentation on installing ICP in a Kubernetes cluster

The ICP documentation(https://mi.docs.wso2.com/en/latest/observe-and-manage/working-with-integration-control-plane/) mentions the configurations to be provided in the ICP side for it to identify the integration nodes(MI or Ballerina). But it does not mention the need of exposing the management API for these nodes in order for node registration to work as expected.

In a k8s environment, I faced an issue where the node registration did not work as expected. For each heartbeat sent from the node, it tries to register again and again because the node registration fails each time.
To overcome this, in a ballerina deployment, I had to specifically expose the management API by exposing the port on which the icp service(https://github.com/ballerina-platform/module-ballerinax-wso2.controlplane/blob/main/ballerina/icpService.bal#L32) is running. Then, we need to define a separate service that exposes this icpService and had to provide that service's URL as the mgtApiUrl which is not clearly mentioned in the documentation.

Start with the linked ICP installation documentation page and review the referenced icpService.bal entry. Document that Kubernetes deployments must expose the integration node management API through a separate service and provide that service URL as mgtApiUrl. Done when the page explains the required exposure and node registration setup clearly.