Fix GitHub Pages documentation rendering

[MaxGhenis]

Summary

Fixes the GitHub Pages documentation site which was showing raw HTML instead of rendered pages.

Problem

The documentation at https://policyengine.github.io/policyengine-us-data/ was not rendering properly because MyST was generating absolute paths (like /build/) that don't work when served from a GitHub Pages subdirectory.

Solution

Set the BASE_URL environment variable to /policyengine-us-data when building documentation for deployment. MyST uses this to generate correct relative paths.

Test

The documentation will render correctly at https://policyengine.github.io/policyengine-us-data/ after this fix is deployed.