Update docs away from split policyengine-us-data state H5 example

從 src/app/page.jsx 開始，檢查它如何建構或記錄州和國會選區的 H5 URI。閱讀 policyengine.py 中的區域/scoping 路徑，然後新增或更新涵蓋州和國會選區執行的回歸測試。當應用程式或文件中的有效參照使用經認證的全國 Populace 資料集，而不是拆分的區域 H5 檔案時，即表示完成。

PolicyEngine/populace#237 removed release generation for one-file-per-state and one-file-per-district artifacts. The replacement path is the single certified Populace national H5 plus the standard region/scoping machinery in policyengine.py, not direct hf://policyengine/policyengine-us-data/states/... or districts/... files.

GitHub code search still finds split-H5 references in this repo:

• src/app/page.jsx

Migration checklist:

• Stop constructing or documenting policyengine-us-data/states/*.h5 and policyengine-us-data/districts/*.h5 dataset URIs.
• Use the certified Populace default dataset through policyengine.py, with region filters/scoping for state and congressional-district analyses.
• Add or update a regression test so future state/CD runs do not depend on split regional H5 files.
• Keep historical/generated output unchanged unless it is actively used by the app or docs.