Introduce a SkrubEstimator to have docs in HTML display

[GaelVaroquaux]

Scikit-learn estimators, and subclasses, have a mechanism to link to the help page (the question mark on the display):

When we subclass in skrub, we must indicate the logic to link to the docs (there is a subclassing mechanism, explained here: https://scikit-learn.org/dev/developers/develop.html#developer-api-for-html-representation).

We should introduce a SkrubEstimator that does this, and systematically inherit from it.

[jeromedockes]

@glemaitre had started working on that in #1051

[glemaitre]

Whoops. I did not remember this one. I'll try to go back to it :)