Update Doc for Managed k8s service

User Story As a Cluster API provider developer, I would like to have a doc describing clear contracts between CAPI and providers for managed k8s service.

Detailed Description When Managed Kubernetes in CAPI proposal is approved, CAPI doc should be updated to reflect the chosen API option.

This is not an exhaustive list and feel free to add items in the comments.

• A new "implementing managed kubernetes" guide that contains details about how to represent a managed Kubernetes service in CAPI. The content will be based on the selected option from the proposal along with other considerations such as managed node and addon management.
• Update the Provider contracts documentation to state that the same kind should not be used to satisfy 2 different provider contracts.
• Update the Cluster Infrastructure documentation to provide guidance on how to populate the controlPlaneEndpoint in the scenario where the control plane creates the api server load balancer. We should include sample code.
• Update the Control Plane Controller diagram for managed k8s services case. The Control Plane reconcile needs to start when InfrastructureReady is true.