Add docs and provide migration script for existing k8s clusters to be able to adopt #432 feature

Created by: matofeder

As a KaaS user, I want to know how to update my existing k8s clusters to be able to adopt: "Add option to use a custom container registry in containerd" #432 feature.

Update project docs and provide migration script for users who want to adopt feature #432 for his/her existing clusters that have been deployed before #432

This issue has been opened based on the comment: https://github.com/SovereignCloudStack/k8s-cluster-api-provider/issues/417#issuecomment-1634428856