Describe the workflow you want to enable

The goal of this issue is to facilitate the discovery of the ability to expand estimators in the HTML repr: people don't realize that they can click on the title of an estimator to expand it.

See for instance on:
https://scikit-learn.org/stable/auto_examples/release_highlights/plot_release_highlights_0_23_0.html

Describe your proposed solution

Add visual cue (using CSS) in form of aright-looking triangle (▸) to the left of the bar that turns down when the object is expanded.

Describe alternatives you've considered, if relevant

The current implementation lacks the visual cue.

Additional context

Slightly related to #21266