Clarify Ch8 - Monads and Applicatives

[milesfrain]

This section could be made more beginner-friendly:
https://book.purescript.org/chapter8.html#monads-and-applicatives

Inspired by #184

Improvement ideas:

1. A concrete example of using ap for an Apply instance.

Replace F with another type

instance applyF :: Apply F where
  apply = ap

https://pursuit.purescript.org/packages/purescript-prelude/4.1.1/docs/Control.Monad#v:ap

2. Note that even though Monad depends on Apply, the implementation of apply can depend on Monad. This flexibility may be unexpected by readers.

3. Change

Try writing userCity using only pure and apply

to

Try writing userCity using only what's available from Applicative (map, apply, pure) and without the additional Bind capabilities of Monad. This means no using bind or <- in do notation.