feat: add docs app with Fumadocs

- Add `docs/` — Next.js 15 app powered by Fumadocs with a landing page, docs layout, search, OG image generation, and LLM text routes
- Cover getting started, configuration, integrations (Plex, Emby, Jellyfin, Radarr, Sonarr), mobile app setup, and telemetry in MDX
- Exclude `docs/` from root Biome config since it has its own `biome.json`

docs/.gitignore

@@ -0,0 +1,26 @@
+# deps
+/node_modules
+
+# generated content
+.source
+
+# test & build
+/coverage
+/.next/
+/out/
+/build
+*.tsbuildinfo
+
+# misc
+.DS_Store
+*.pem
+/.pnp
+.pnp.js
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+
+# others
+.env*.local
+.vercel
+next-env.d.ts