dvclive: example of data format for all plots templates in log_plot

[dberenbaum]

Another thought on a lightweight way to help here: better docs in https://dvc.org/doc/dvclive/live/log_plot. Having an example of the input format for each template could go a long way. There are already examples of different templates in https://dvc.org/doc/command-reference/plots/show that we could use as a starting point.

Originally posted by @dberenbaum in https://github.com/iterative/dvclive/issues/750#issuecomment-1845865297