Documentation about how to use custom resources

[janiskemper]

/kind feature

Describe the solution you'd like
There exists the https://github.com/SovereignCloudStack/cluster-stacks-demo that makes it easy with two make targets to create a workload cluster with docker.

However, there is no documentation yet on how the actual process of dealing with the cluster-stack-operator looks like.

1. Create ClusterStack and maybe the ProviderClusterStackReleaseTemplate
2. Wait until a release of a cluster stack is ready
3. Create a cluster with the ClusterClass feature of Cluster API by referencing the ClusterClass corresponding to the respective version of a cluster stack.

These three steps should be documented appropriately. Additionally, there should be some hints on where to look in order to get more information about the system.