restore documentation publishing and modernize CI

[janzill]

Restore Sphinx builds in GitHub Actions, with documentation and PyPI publishing triggered only by published GitHub releases. Standardize builds around uv and locked dependencies, remove develop-branch assumptions, and update README and documentation for the current architecture and uv setup.

Would require the following one-time setup:

1. In ActivitySim/populationsim, open Settings → Pages and select GitHub Actions as the publishing source. GitHub instructions
2. Create or review the github-pages and pypi environments. If deployment restrictions are enabled, allow tags matching v* in both environments. Allowing only the master branch will block release deployments. Environment configuration
3. On PyPI, open the populationsim project’s Publishing settings and add a GitHub Actions trusted publisher with:
   • Owner: ActivitySim
   • Repository: populationsim
   • Workflow filename: release.yml
   • Environment: pypi
     No PyPI API-token secret is required. PyPI instructions
     Release procedure:
4. Update pyproject.toml to a new version, run uv lock, and merge the changes to master.
5. Manually create and publish a GitHub release targeting that commit, with a matching tag—for example, version 0.10.1 and tag v0.10.1.
6. Confirm the Release and Documentation workflows succeed, then verify PyPI and the documentation site.
   Master changes and pull requests run validation only. Pushing a tag or saving a draft release does not publish.