Fix docs package index deployment

[eric-tramel]

What

Update the Documentation workflow so deploy builds can reliably include the released package index metadata.

Why

The data-designer-retrieval-sdg v0.1.0 release successfully published wheel and sdist assets, but the main docs deployment rebuilt Pages with an empty package list. That left the public Simple index empty even though the release artifact bucket had packages.json.

How

• Download ddp-package-assets/packages.json by resolving the release asset through the GitHub API instead of relying on the lossy gh release download fallback.
• Log the package-list byte count so empty deploys are visible in CI logs.
• Allow workflow_dispatch runs on main to configure, upload, and deploy Pages, which gives maintainers a safe manual recovery path after package asset changes.

Validation

• Parsed .github/workflows/docs.yml with PyYAML.
• Verified the new gh api asset download returns the 338-byte released packages.json.
• Ran make docs PACKAGE_LIST= locally and confirmed site/simple/data-designer-retrieval-sdg/index.html and pypi JSON files are generated.