Document supported Kubernetes versions

[siegfriedweber]

This ticket should add a section to the end user documentation about the decision taken in stackabletech/issues#113.

It should be a quick summary of the policy but should also list concrete versions.

[berndfo]

content will be merged here https://github.com/stackabletech/documentation/blob/adr-supported-kubernetes-versions.adoc/modules/adr/pages/drafts/ADRnnn-supported-kubernetes-versions.adoc