ci(docs): derive pnpm store path and bump actions/cache to v6
Review follow-up:
- Resolve the pnpm store path via `pnpm store path` instead of hardcoding
~/.local/share/pnpm/store (it is actually .../store/v11 and varies by pnpm
version / XDG config, so the hardcoded key would never hit).
- Bump actions/cache v4 -> v6 (latest major).
pnpm stays on @latest via corepack by request (always track the newest release).