[generics-rep] Add expanded documentation

[milesfrain]

The purescript-contrib library guidelines suggest an expanded documentation directory. If we follow that same structure for the core libs (like generics-rep), then I'd like to add these guides on how to derive from generic:
purescript/documentation@9409b9a
purescript/documentation@bb30b15

That content was originally proposed for the language reference in purescript/documentation#338