Drop docker/k8s-first-steps as unmaintained

[eemeli]

We've not really done anything with Kubernetes/k8s since #1558 / #1580, leaving the docker/k8s-first-steps folder and its contents effectively unmaintained. We also have no current plans for doing anything with these.

For the sake of clarity, we should drop this from the repo as I'm not sure we've ever even tested if the example config works.