Replace the static one-page index.html with a Node-based docs site
generator. Sidebar nav, per-page TOC, dark mode toggle (system-pref
default, persisted), and a Spotify-green theme with animated equalizer
brand mark and ambient hero gradient.
Adds 11 hand-written doc pages (index, install, quickstart, auth,
playback, library, queue, devices, engines, output, agents,
troubleshooting, commands) alongside existing spec.md and RELEASING.md.
Pages workflow deploys dist/docs-site to GitHub Pages on push to main;
repo Pages source is now "GitHub Actions" instead of legacy /docs.
