extend generics slides to explain what to put where when

[Lotterleben]

https://ferrous-systems.github.io/teaching-material/generics-basics.html#/

introduces them in a very basic way, but the thing i keep tripping over and I've also seen participants and pairing partners trip over is where to put which <>s in more complex scenarios (impl of a generic-ish trait for a struct that golds generic members or something like that)