feat(docs): add documentation site and demo features

- Introduced a new documentation site for Cursor BYOK using Next.js and Fumadocs.
- Added a product demo page with a corresponding Vite configuration.
- Implemented a demo API to simulate LLM calls and responses.
- Enhanced the Makefile to include new build and development commands for the documentation.
- Updated package.json scripts for building and running the documentation site.
- Created various components and layouts for the documentation structure, including blog and user documentation sections.
- Added styling for the new components and layouts to ensure a cohesive design.
- Included a README and other necessary files for local development and deployment.

apps/docs/components/hero/DesktopDemo.tsx

@@ -0,0 +1,18 @@
+'use client';
+
+import styles from './DesktopDemo.module.css';
+
+export function DesktopDemo() {
+  return (
+    <div className={styles.stage}>
+      <div className={styles.viewport}>
+        <iframe
+          title="Cursor BYOK 真实产品界面演示"
+          src="/product-demo/demo/index.html"
+          className={styles.demo}
+        />
+      </div>
+      <p>真实桌面端组件 · 使用隔离的 Mock 数据，可直接操作菜单、筛选和设置</p>
+    </div>
+  );
+}