Better Documentation for Types

[KodrAus]

Add more useful docs to module roots in the types crate. This should include examples of how to define types with default mappings, create custom mapping etc.

Base the docs around what users are likely going to want to do, and provide alternatives and details on the method docs themselves.

[KodrAus]

Once #59 is implemented, there should be enough to cover some standard scenarios in the docs.

[KodrAus]

Waiting on #65

[KodrAus]

This is improving in general along with the module design

[KodrAus]

Pick this issue up again once the core types are implemented so the docs can be worked on as a whole.

[KodrAus]

The state of the types documentation is much better now.

Examples of building types with compiler plugins will be added as they're done as per normal.