README: simplify

[casperdcl]

Part of #514

• Adhere to READMEs vs Docs dvc.org#2443 in general
• Migrate command details to cml.dev/doc/ref
• Anything else?

[jorgeorpinel]

Is this p1 since it's listed in iterative/cml.dev#250 ? I propose we focus on this first for the high-level docs/prod work (see roadmap).