Wire generated reference docs into checks

[MaxGhenis]

Summary

• add a Reference landing page to the Quarto docs
• add Make targets for full US reference generation and a fast CHIP-filtered smoke build
• generate program reference pages from programs.yaml and link them to implementation variables
• ignore generated reference output under docs/_generated
• run the reference-generator smoke build in docs CI before rendering the docs site

Test

• ruff check docs/_generator/build_reference.py
• ruff format docs/_generator/build_reference.py --check
• make docs-reference-smoke
• make docs
• manual GitHub Docs changes run: https://github.com/PolicyEngine/policyengine.py/actions/runs/25016292214