Make references to scikit-learn examples more visible

[ArturoAmorQ]

Currently we have links to the scikit-learn example gallery inside the "Main take-away" notebook at the end of each module (see for instance here) and possibly they are being overlooked.

For the sake of visibility, we can consider moving such links to a "To go further" section at the end of some notebooks whenever relevant.

[lesteve]

Maybe doing both would be good, but then there is a bit of copying and pasting that can go out of sync ... unless we want to add yet another hacky script to our setup ...