Add comprehensive example notebook demonstrating RiskModel class

The notebook covers:
- Setup and data loading
- Portfolio, benchmark, and active weight visualization
- Factor exposures and covariance/correlation matrices
- Portfolio-level risk measures (factor, specific, total)
- Stock-level risk attribution (MCTR, CTR, PCTR)
- Factor-level risk decomposition
- Summary dashboard
- Appendix with raw risk matrices and factor returns

Author: claude