Add blog post explaining OBBBA modeling methodology

[MaxGhenis]

Summary

Create a comprehensive blog post/report explaining how PolicyEngine models the Old-Age, Blind, and Disabled Basic Assistance (OBBBA) program.

Content to Include

The report should cover:

1. Policy Rules Captured

   • Eligibility criteria we model
   • Benefit calculation methodology
   • Interaction with other programs (SSI, etc.)
   • Any simplifications or assumptions made

2. Data Sources

   • Primary data sources used for OBBBA modeling
   • How we handle data limitations
   • Any imputation methods used

3. Calibration Methodology

   • How we calibrate benefit losses to CBO's projections
   • Specific CBO estimates we target
   • Calibration parameters and their values
   • Validation of calibration results

4. Technical Implementation Details

   • Key variables and parameters in the model
   • Any special handling or edge cases

Rationale

This documentation will help users understand:

• The accuracy and limitations of our OBBBA modeling
• How our estimates align with official projections
• The methodological choices we've made

Suggested Format

• Blog post in the PolicyEngine blog section
• Technical but accessible to policy analysts
• Include relevant charts/visualizations
• Link to relevant code/parameters in policyengine-us

Related to the OBBBA household-by-household calculator and recent URL sync fixes.