Have a stylesheet documentation

It's not easy to guess which CSS classes are available in Adwaita, what they affect, or when to use them, and it have been hurting some developers. Currently style classes are documented either in the CSS node section of a relevant object's documentation, or are simply undocumented.

It should be easier to find them and to know what they do, probably with a stylesheet-specific documentation. Given libadwaita provides the Adwaita stylesheet, a centralized stylesheet-specific documentation likely should live in libadwaita's documentation, and the HIG could point to it when relevant.