Gallery UI improvements

There are a few improvements I'd like for the gallery.

First, I'd love to have collapsible sections in the menu, using something like https://jqueryui.com/accordion/

Second, there should be a description for each entry, which is shown beside or below the canvas when it is clicked. Once that's done, the optional source links added by @nomeata can be moved into the description rather than the button.

Start by locating the gallery menu, its entry data, and the canvas click behavior. Add collapsible menu sections and display an entry description beside or below the selected canvas, moving optional source links into that description; verify both interactions in the gallery.

Rédigé par le modèle d'indexation à partir du texte de l'issue.