Document how to install on microk8s

[nightkr]

• Enable the hostpath-storage (or another storage provider) and dns microk8s addons
• Pass kubeletDir Helm value for secret-op ([Merged by Bors] - Make kubelet dir configurable (fixes #229) secret-operator#232) and listener-op (Support microk8s listener-operator#44)

[fhennig]

Related:

• Expand explanation for k8s setup #205
• Documentation: Which K8S Products and Versions do we support? #261

We should make a dedicated Kubernetes page to document this, and other things from the tickets above