Added K3s guidance for installing docker container runtime

[adamhurm]

I added some content to the K3s page to assist with setup:

• docker is needed for some of the scenarios, so I have added instructions for installing. The K3s default is containerd.
• docker uses systemd cgroup by default, but K3s does not support it due to dependencies on statically-linked binaries.

[madhuakula]

Thanks for taking the initiative and adding this doc, really appreciate it @adamhurm 🎉