Enhance Decision Tree documentation with math, use cases, and examples

- Added comprehensive mathematical foundation explaining MSE and tree splitting
- Included practical use cases (house prices, stock forecasting, customer lifetime value, etc.)
- Listed advantages and limitations of decision trees
- Enhanced class and method docstrings with detailed explanations
- Added type hints for better code clarity
- Included additional practical example (house price prediction)
- Improved code documentation following repository guidelines

Related to #12867

Author: Tejasrahane