feat: Add comprehensive docstrings to Domain and Factor classes

[ryan112358]

Adds detailed docstrings to the Domain class in src/mbi/domain.py and the Factor class in src/mbi/factor.py.

The docstrings explain the purpose of each class, their parameters, and key methods. This improves code understanding and maintainability. Focus was placed on the class-level docstrings and core functionalities based on user feedback during implementation.