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/app/layout.tsx

@@ -0,0 +1,26 @@
+import { RootProvider } from 'fumadocs-ui/provider/next';
+import { i18nProvider } from 'fumadocs-ui/i18n';
+import type { Metadata } from 'next';
+import { translations } from '@/lib/layout.shared';
+import './global.css';
+
+const siteUrl = process.env.NEXT_PUBLIC_SITE_URL ?? 'https://docs.leokun.cn';
+
+export const metadata: Metadata = {
+  metadataBase: new URL(siteUrl),
+  title: {
+    default: 'cursor-byok 文档',
+    template: '%s | cursor-byok',
+  },
+  description: 'cursor-byok 的安装、模型配置与故障排查指南。',
+};
+
+export default function Layout({ children }: LayoutProps<'/'>) {
+  return (
+    <html lang="zh-CN" suppressHydrationWarning>
+      <body className="flex min-h-screen flex-col">
+        <RootProvider i18n={i18nProvider(translations)}>{children}</RootProvider>
+      </body>
+    </html>
+  );
+}