Document new hidden provisioner feature

[richard-cox]

• See Hide AKS, EKS, and GKE cards when their kontainer drivers are deactivated #12903
• We need to document the ability of extension providers to hide their cluster type
• This is particularly important for hosted/cluster driver based cluster's to implement to hide if their associated kontainer driver CR is missing

Would this document need to be aligned with the change existing in a shell release?