DataTree: Auto-plotting capabilities

Is your feature request related to a problem?

Xarray is already capable of smart plotting data arrays and in some extents, datasets.

Xarray has limited support for plotting Dataset variables against each other.

https://docs.xarray.dev/en/latest/user-guide/plotting.html

Describe the solution you'd like

The rich display tree structure could be reused with a default .plot() call inside of it, for example.

However, it is also a change that would concern Datasets themselves as there is no way currently plot all the variables of a Dataset.

Experience feedback: oftentimes I convert a Dataset to a DataArray with ds.to_array() for plotting with facets. This is possible when variables of the dataset have the same dimensions.

Describe alternatives you've considered

Making visualization functions myself. However, the capacity to "just plot" data is often welcomed!

Additional context

Issue content taken from https://github.com/xarray-contrib/datatree/issues/336

Start with the xarray plotting documentation and the existing .plot() and ds.to_array() entry points mentioned in the issue. Determine how Dataset variables and the DataTree rich display should interact, then define the expected behavior for plotting variables against each other and confirm it with appropriate coverage.