Replacing `CUSKs` examples with `StandaloneKindSignatures` throughout Kind Polymorphism's documentation

Summary

Location of documentation issue: GHC user's guide, Kind Polymorphism

Even thought CUSKs is now considered legacy, it is used throughout the kind polymorphism section a number of times as an example.

This example is from kind inference in closed type families:

type family F1 a where
  F1 True  = False
  F1 False = True
  F1 x     = x
-- F1 fails to compile: kind-indexing is not inferred

type family F2 (a :: k) where
  F2 True  = False
  F2 False = True
  F2 x     = x
-- F2 fails to compile: no complete signature

type family F3 (a :: k) :: k where
  F3 True  = False
  F3 False = True
  F3 x     = x
-- OK

Proposed improvements or changes

I think there are 3 good options:

• Remove them and rewrite them using StandaloneKindSignatures
• Add equivalent examples using StandaloneKindSignatures, comment the original CUSKs examples, and add a little note to them explaining that they are legacy
• Add the examples using StandaloneKindSignatures, and add a short comment to the CUSKs examples, stating that they are legacy