Make references to scikit-learn examples more visible

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](https://inria.github.io/scikit-learn-mooc/predictive_modeling_pipeline/predictive_modeling_module_take_away.html)) 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.