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/next.config.mjs

@@ -0,0 +1,23 @@
+import { createMDX } from "fumadocs-mdx/next";
+
+const withMDX = createMDX();
+
+/** @type {import('next').NextConfig} */
+const config = {
+  serverExternalPackages: ["@takumi-rs/image-response"],
+  reactStrictMode: true,
+  turbopack: {
+    // docs site is essentially self-contained from the rest of the monorepo
+    root: import.meta.dirname,
+  },
+  async rewrites() {
+    return [
+      {
+        source: "/docs/:path*.mdx",
+        destination: "/llms.mdx/docs/:path*",
+      },
+    ];
+  },
+};
+
+export default withMDX(config);