Add examples gallery

[Kochise]

To quickly find what it looks like

[MatthieuDartiailh]

Th documentation already include screenshots of each examples cf https://enaml.readthedocs.io/en/latest/examples/ex_buttons.html

[Kochise]

Yeah, it was about to have it locally (thumbnail explorer is more convenient).

Or add an exhaustive gallery on the readthedocs.io page.