Add derive via Generic example, and point to latest docs

I often visit this page looking for a `genericShow` example.

It was also frustrating to click on the `Data.Generic.Rep` and be taken to an older package version that's missing lots of nice documentation. So ensuring links now go to the latest version. (related to purescript/pursuit#414)

language/Type-Classes.md

@@ -147,11 +147,22 @@ derive instance ordPerson :: Ord Person
 ```
 Currently, the following type classes can be derived:
 
-- [Data.Generic.Rep (class Generic)](https://pursuit.purescript.org/packages/purescript-generics-rep/6.0.0/docs/Data.Generic.Rep#t:Generic)
-- [Data.Eq (class Eq)](https://pursuit.purescript.org/packages/purescript-prelude/4.1.0/docs/Data.Eq#t:Eq)
-- [Data.Ord (class Ord)](https://pursuit.purescript.org/packages/purescript-prelude/4.1.0/docs/Data.Ord#t:Ord)
-- [Data.Functor (class Functor)](https://pursuit.purescript.org/packages/purescript-prelude/4.1.0/docs/Data.Functor#t:Functor)
-- [Data.Newtype (class Newtype)](https://pursuit.purescript.org/packages/purescript-newtype/3.0.0/docs/Data.Newtype#t:Newtype)
+- [Data.Generic.Rep (class Generic)](https://pursuit.purescript.org/packages/purescript-generics-rep/docs/Data.Generic.Rep#t:Generic)
+- [Data.Eq (class Eq)](https://pursuit.purescript.org/packages/purescript-prelude/docs/Data.Eq#t:Eq)
+- [Data.Ord (class Ord)](https://pursuit.purescript.org/packages/purescript-prelude/docs/Data.Ord#t:Ord)
+- [Data.Functor (class Functor)](https://pursuit.purescript.org/packages/purescript-prelude/docs/Data.Functor#t:Functor)
+- [Data.Newtype (class Newtype)](https://pursuit.purescript.org/packages/purescript-newtype/docs/Data.Newtype#t:Newtype)
+
+Other type classes may be derived via `Generic`:
+```purescript
+import Data.Generic.Rep (class Generic)
+import Data.Generic.Rep.Show (genericShow)
+
+derive instance genericPerson :: Generic Person _
+
+instance showPerson :: Show Person where
+  show = genericShow
+```
 
 ## Compiler-Solvable Type Classes