Document example of a Mixin

[BassMastaCod]

Add documentation, and perhaps a useful example, of how to use a DAOModel mixin. Something like:

class AuditableMixin:
    """Mixin that adds created_at, updated_at, and deleted_at fields."""
    created_at: datetime = CreationTimestampField
    updated_at: datetime = AutoUpdatingTimestampField
    deleted_at: Optional[datetime] = Field(default=None)

Bassically, reuseable code that covers common model boilerplate fields or functionality.