datatree: Collapsible items in `groups` DataTree

[owenlittlejohns]

What is your issue?

Originally posted by @agrouaze in xarray-contrib/datatree#145
Attempted implementation in this PR

Having collapsible items (like xarray.Datasets) in groups repr_html_ would help to have user friendly overview of Python objects.
I am wondering whether this feature is already available or not.

Clarification: The ask here is to make individual groups collapsible in the HTML rendering, so it is possible to see all the child groups, without having to see the full contents of a group.

[owenlittlejohns]

When migrating this issue, we discussed that this work may need to (or wait for) be part of a larger rewrite of the HTML rendering code.

[OriolAbril]

Is there anything that can be done to help? I am not very familiar with HTML but very keen on helping if possible. We would like to switch to DataTree at ArviZ instead of using InferenceData and this will be an important friction point as we'll generally have nothing in the root group and what we care about is seeing the groups then inspecting the contents of some of them.