Added Fama-French 3-factor and 5-factor expected return models

[WilliamTDavies]

Closes #595
Closes #634

Summary

Adds support for Fama-French factor-based expected return estimation.

New functionality

• Added Fama-French 3-factor expected return model.
• Added Fama-French 5-factor expected return model.
• Added support through return_model() via:
  • ff3_return
  • ff5_return

Documentation

• Added API documentation for Fama-French expected return estimation.
• Updated expected returns documentation.
• Updated README expected returns section.

Tests

Added tests covering:

• FF3 and FF5 expected return estimation.
• return_model() dispatch.
• Invalid model selection.
• Missing required factor columns.
• Non-overlapping asset and factor data.
• Fully missing aligned factor data.
• returns_data=True support.
• Compounding and non-compounding annualisation.
• Validation against synthetic data with known factor exposures.
• Handling of additional unused factor columns.