Revamp the developer documentation when it comes to roll scikit-learn compatible estimator

[glemaitre]

I find the documentation helping at writing a scikit-learn estimator a bit oldish: https://scikit-learn.org/dev/developers/develop.html

I think that we could revamp the documentation with a new look. Probably, we would like to mention what are the minimum implementation required and then go into details in the additional feature given by the mixin that we added overtime.

Finally, it should be the place where we provide some documentation regarding the developer tools and manage the expectation regarding the deprecation cycle for those.