[k8s][docs] K3s docs and runtime auto-detection

[romilbhardwaj]

Closes #3034 and #2958. Adds docs and automatic runtimeclass detection for k3s users.

Tested (run the relevant ones):

• Docs rendered locally
• Code formatting: bash format.sh
• Tested GPU labelling script and sky launch on a manually setup k3s cluster
• Tested sky launch on GKE cluster
• Kubernetes smoke tests using T4.

[Michaelvll]

This is awesome @romilbhardwaj! Left some nits.

[Michaelvll]

Thanks for the quick fix @romilbhardwaj! LGTM.

[romilbhardwaj]

Thanks @Michaelvll! I've refactored and tested the gpu_labeller on k3s and EKS.

I've also added a deploy_k3s.sh for our debugging purposes to easily deploy a k3s cluster for testing.