docs: document buildkit drivers specifically kubernetes driver

[sozercan]

we should document buildkit drivers, specifically the kubernetes one for users that want to build inside k8s
https://docs.docker.com/build/builders/drivers/
https://docs.docker.com/build/builders/drivers/kubernetes/

[cpuguy83]

Why do we want to document stuff about docker?

[sozercan]

@cpuguy83 i think people aren't going to know about this, so i thought we'll leave a small breadcrumb