Document randomizable types

[sunjay]

For every type that can be randomized (Color, Point, Speed, etc.) we should add a section Generating Random X that documents how to use random() and random_range() to generate values of that type.

• Behaviour of random() and random_range() for each type
• Examples of using each
• Examples of the different bounds types supported by each (most of them implement RandomRange<B> with B: Into<X>)
• Do this for the scalar types too (Distance, etc.)