[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: https://github.com/purescript/documentation/pull/338/commits/9409b9a047aaa176199053928177c92739566a72 https://github.com/purescript/documentation/pull/338/commits/bb30b1514c406ee24eb6796d86ba282de323c4b7

That content was originally proposed for the language reference in https://github.com/purescript/documentation/pull/338