Add comprehensive documentation to all functions

Enhance code quality with docstrings, type hints, and inline comments:

- is_numeric_answer: Type hints (str -> bool) + docstring explaining validation logic
- init_dataset: Full type hints + comprehensive docstring covering all parameters, returns, and raises
- metric: Type hints + docstring explaining evaluation logic and return values
- metric_with_feedback: Type hints + detailed docstring explaining GEPA feedback generation

All functions now include:
- Google-style docstrings with Args, Returns, and Raises sections
- Type hints for parameters and return values
- Inline comments explaining key logic steps
- Clear parameter descriptions and default values

Improves code readability, maintainability, and serves as educational reference for DSPy users.

Author: behroozazarkhalili