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/source.config.ts

@@ -0,0 +1,23 @@
+import { metaSchema, pageSchema } from "fumadocs-core/source/schema";
+import { defineConfig, defineDocs } from "fumadocs-mdx/config";
+
+// You can customise Zod schemas for frontmatter and `meta.json` here
+// see https://fumadocs.dev/docs/mdx/collections
+export const docs = defineDocs({
+  dir: "content/docs",
+  docs: {
+    schema: pageSchema,
+    postprocess: {
+      includeProcessedMarkdown: true,
+    },
+  },
+  meta: {
+    schema: metaSchema,
+  },
+});
+
+export default defineConfig({
+  mdxOptions: {
+    // MDX options
+  },
+});