Add Documentation for Let

The Let family of exercises (graphics, bool, sequence, base (?)) currently has only little, outdated documentation in #661 (closed)

The Readme should describe the outline of this module a bit more verbosely and more functions need documentation.

What I could grasp from reading the source:

Let.Top defines a Partial instance for all types with a Domain instance.

The requirements for an instance of the Domain class are a bit complicated and hardly any of the classes methods have documentation

The Domain instance for Bool does a lot of stuff that makes sense upon first look, but also defines new type aliases to the unit type that are not as easy to grasp.

The same is true for the Domain instance for Pic.

In general there are a LOT of types, type classes and type class instances declared in this package, and some seem pretty similar (e.g. Exp vs. ExpFun)