Add fresh methodology and program docs scaffold

[MaxGhenis]

Summary

• add Methodology and Programs sections to the Quarto docs navigation
• add a from-scratch model architecture page describing the split between authored methodology, program pages, and generated reference
• add a fresh US health coverage/costs methodology page verified against the pinned policyengine-us dependency
• add a fresh US CHIP program page that explains structure and delegates exact variable/parameter listings to generated reference

Notes

This is not a straight port of policyengine-us/docs or the docs-quarto prototype. I used those pages only as reference material and rewrote the docs around the structure we would want if starting fresh: authored pages explain model choices; generated reference should own variable, parameter, and data-lineage details.

Verification against policyengine-us==1.667.1 affected the health page: current release behavior is described separately from upcoming Marketplace premium/resource-integration work.

Test

• make docs
• uv run --extra us python checks for referenced US variables