ci: publish PEP 503 package index for release wheels

[Shiti]

Summary

uniko's Python wheels are distributed as GitHub release assets (each is ~120 MB, over PyPI's 100 MB per-file limit), so they cannot be installed from PyPI.

Publish a static PEP 503 "simple" index on the docs GitHub Pages site so pip/poetry can install them via --extra-index-url https://rustic-ai.github.io/uniko/packages/.

• scripts/ci/gen_package_index.py: generate the index from the GitHub releases API, grouping every wheel/sdist under its normalized project name (uniko, uniko-cuda, uniko-metal) and pinning #sha256 from asset digests (no downloads; stdlib only).
• deploy-docs.yml: regenerate the index before building the site, and expose workflow_call so the release can invoke it. The generated tree (website/docs/packages/) is gitignored — the releases API is the source of truth.
• release.yml: after github-release attaches the wheels, call deploy-docs to rebuild + redeploy so a new release is immediately installable. Invoked directly because GITHUB_TOKEN-created releases don't cascade into trigger-based workflows.

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would change existing behavior)
• Refactor (no functional change)
• Documentation
• CI / build / tooling

Checklist

• cargo fmt --check passes
• cargo clippy -- -D warnings passes
• cargo nextest run passes
• cargo deny check passes (licenses / advisories)
• uni-db seal respected (product crates do not use uni_db or call .db() directly — access goes through uniko-store)
• Documentation updated where relevant

Linked issues