Reduce CML with DVC report generation code

Related to #389.

In https://cml.dev/doc/cml-with-dvc, the "Create CML report" step is more than half the YAML file for both GH and GL. I think it's a useful example but makes CML<>DVC integration look way more intimidating than it needs to be.

Some initial ideas for improvement:
1. Move that step to a separate section further down the page to keep the intro example more concise.
2. Show a DVCLive example, which will generate the markdown report on its own. Also, should the CML<>DVCLive live update workflow be documented somewhere in this page?
3. Use `cml pr` and mention that you can view DVC metrics and plots in Studio (related: https://github.com/iterative/studio/issues/4232).