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/desktop/demo/index.html

@@ -0,0 +1,23 @@
+<!doctype html>
+<html lang="zh-CN">
+  <head>
+    <meta charset="utf-8" />
+    <meta name="viewport" content="width=device-width, initial-scale=1" />
+    <meta name="theme-color" content="#141414" />
+    <style>
+      html,
+      body,
+      #root {
+        width: 100%;
+        height: 100%;
+        margin: 0;
+        background: #141414;
+      }
+    </style>
+    <title>Cursor BYOK 产品演示</title>
+  </head>
+  <body>
+    <div id="root"></div>
+    <script src="/src/demo/index.tsx" type="module"></script>
+  </body>
+</html>