Document new command-line interface structure

The following pull request have introduced some drastic naming changes that should be documented:
* https://github.com/iterative/cml/pull/1026
* https://github.com/iterative/cml/pull/1073

> **Note**
> Closing this issue won't only require updating the documentation, but also all the active example repositories and material.

- [x] https://cml.dev/ #324
+ [x] https://cml.dev/doc/
* [x] https://cml.dev/doc/ref/
- [x] runner
- [x] comment
- [x] check
- [x] pr
- [x] workflow
- [x] tensorboard
* [ ] DVCLive demo for `--watch --publish` #327
- [ ] cml readme
- [ ] example repos https://github.com/iterative/cml-examples-dev/issues/9
+ [ ] [GH](https://github.com/iterative?q=topic%3Aexample+cml&type=source)
+ [ ] GL
+ [ ] BB
- [x] https://github.com/iterative/cml/labels