Restructure modules and add documentation

[patrick91]

See #3542

• [x] strawberry.auto -> strawberry.types.auto
• [x] strawberry.private -> strawberry.types.private
• [x] strawberry.enum -> strawberry.types.enum
• [x] strawberry.union -> strawberry.types.union
• [x] strawberry.lazy_type -> strawberry.types.lazy
• [x] strawberry.unset -> strawberry.types.unset
• [x] strawberry.custom_scalar -> strawberry.types.scalar
• [ ] strawberry.object_type -> strawberry.types.object_type
• [ ] strawberry.type -> strawberry.types.type
• [ ] strawberry.field -> strawberry.types.fields.field
• [ ] strawberry.mutation -> strawberry.types.fields.mutation
• [ ] strawberry.arguments -> strawberry.types.fields

@bellini666 I've only added the ones that will go into the types module for now, I wonder if we should also move things like directives to the types module?