Add example usage cases for `cml ci`

[dacbd]

If you allow me to chime in, from the user perspective I think usage cases are way clearer than just random notes.

For instance, I understand these fixes are only applicable to GitHub Actions, but in my opinion there is no clear documentation on CML CI which I think won't be normally installed since docker containers or virtual machines are normally the most common way to deal with this part.

Originally posted by @datacubeR in iterative/dvc#7547 (comment)