ci: make docs workspace self-contained for Vercel

[TrueNine]

Summary

• add a docs-local pnpm workspace catalog for the docs app
• generate a docs-local pnpm lockfile so Vercel detects pnpm instead of npm
• keep the docs app installable without relying on the repo-root workspace metadata

Testing

• pnpm -C doc install --lockfile-only --ignore-scripts
• pnpm -C doc run lint
• cargo run -p xtask -- doc-build