DOC make the link between examples and code on the level of methods and not whole object

Basically, split the small gallery at the bottom of https://tree-timc.github.io/circhic/modules/generated/circhic.CircHiCFigure.html#circhic.CircHiCFigure

between each method. Dunno if it is possible -- will have to explore method's templates? Matplotlib may be doing this already.