Add user docs site, publish under /docs/ on Pages

[mode777]

What

• New user-docs/ VitePress site (10 pages, isolated deps + committed lockfile): getting started, browsing, playlists & favorites, player & formats, adding music, hosting, administration, troubleshooting, FAQ. Dark theme matching the app/landing palette; local search; base path /trackify/docs/ via DOCS_BASE_PATH.
• .github/workflows/docs.yml now installs + builds the docs (npm cache keyed on the docs lockfile), composes them under _pages/docs/ beside the landing, and extends the private-hostname leak scan to user-docs/.
• Landing wired as the docs front door: Docs button in nav, "Read the Docs" hero CTA, hosting/admin links in the self-host band, docs links in the footer.
• .gitignore: ignore VitePress dist/cache.

Why

Second half of the PSflix-pattern rollout (OpenSpec change add-user-docs): operator-facing documentation for running an instance, managing the catalog, and daily use.

How to test

• PR CI: docs build + leak scan + landing link check run without deploying.
• Locally: cd user-docs && npm ci && npm run build → /trackify/docs/ asset paths; composed artifact serves /index.html and /docs/index.html with all four docs/… landing refs resolving.
• Operator walkthrough (no-Docker variant) executed against a live local PocketBase: /, /api/health, /admin.html, /_/, /wasm/backend_psx.js all 200.