[DOC] code snippets and minigallery in user guide pages

[jpaillard]

• As mentioned in [DOC] Section 2: add DL and KO #558 should we systematically add a minigallery of examples using the method described in the user guide pages?
• If yes, should it be at the end of the page or near the top?
• Also, regarding code snippets, should we place them near the top of the pages or at the end? The pages of the user guide can be quite technical. The question is whether we should showcase how to use the method before providing a technical description of it.

[bthirion]

At the end, like sklearn.

I think that code snippets should come along with the explanations.