Allow Custom Indexes to define HTML repr for managed coords

Is your feature request related to a problem?

When a complicated index is attached to a dataset it can potentially fill up the coords field with lots of extra low value information. For example for the builtin multiindex the xarray version looks like this:

which is lower signal to noise than the equivalent pandas repr:

or in the case of layered intervals annotating a dimension (https://ianhuntisaak.com/xarray-linked-indexes/) the coords can really get out of hand

Describe the solution you'd like

Allow Indexes to override the default html repr for any coords that they manage. The dataset repr can compose itself out of the reprs for coords from coords, so the would be in charge of returing the html +/- css for those coords

I think the built in multindex could be a nice example use case of this.

Describe alternatives you've considered

No response

Additional context

No response

cc @benbovy @keewis @dcherian

Start by tracing how Dataset reprs compose HTML for managed coordinates, then compare the existing xarray and pandas MultiIndex representations. The issue's target is an Index-level HTML repr that the Dataset can compose for its managed coords; done means custom indexes can reduce excessive coordinate output while preserving useful information.