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.module.css

@@ -0,0 +1,45 @@
+.stage {
+  width: 100%;
+  max-width: 960px;
+  margin: 64px auto 0;
+}
+
+.viewport {
+  position: relative;
+  width: 100%;
+  aspect-ratio: 960 / 620;
+  overflow: hidden;
+  background: #141414;
+  border: 1px solid rgb(255 255 255 / 10%);
+  border-radius: 14px;
+  box-shadow:
+    0 42px 90px -34px rgb(0 0 0 / 50%),
+    0 12px 32px -16px rgb(0 0 0 / 42%);
+}
+
+.demo {
+  position: absolute;
+  inset: 0;
+  display: block;
+  width: 100%;
+  height: 100%;
+  background: #141414;
+  border: 0;
+}
+
+.stage p {
+  margin: 14px 0 0;
+  color: var(--color-fd-muted-foreground);
+  font-size: 12px;
+  text-align: center;
+}
+
+@media (max-width: 680px) {
+  .stage {
+    margin-top: 42px;
+  }
+
+  .viewport {
+    border-radius: 10px;
+  }
+}

apps/docs/components/hero/DesktopDemo.tsx

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+'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>
+  );
+}