docs: flatten navigation structure by removing language folder hierarchy

- Remove the '文档' parent category from sidebar
- Display all documentation pages directly in the navigation
- Simplify user experience by eliminating unnecessary directory structure

Author: loveTsong

_includes/sidebar.html

@@ -2,19 +2,14 @@
   <h3>目录</h3>
   <ul>
     <li><a href="{{ '/' | relative_url }}">首页</a></li>
-    <li>
-      <a href="{{ '/docs/zh_CN/' | relative_url }}">文档</a>
-      <ul>
-        <li><a href="{{ '/docs/zh_CN/model-access/' | relative_url }}">模型接入</a></li>
-        <li><a href="{{ '/docs/zh_CN/build-agent/' | relative_url }}">构建智能体</a></li>
-        <li><a href="{{ '/docs/zh_CN/build-conversational-assistant/' | relative_url }}">构建对话助手</a></li>
-        <li><a href="{{ '/docs/zh_CN/workflow/' | relative_url }}">工作流</a></li>
-        <li><a href="{{ '/docs/zh_CN/knowledge-base/' | relative_url }}">知识库</a></li>
-        <li><a href="{{ '/docs/zh_CN/plugins/' | relative_url }}">插件</a></li>
-        <li><a href="{{ '/docs/zh_CN/forms/' | relative_url }}">表单</a></li>
-        <li><a href="{{ '/docs/zh_CN/application-market/' | relative_url }}">应用市场</a></li>
-      </ul>
-    </li>
+    <li><a href="{{ '/docs/zh_CN/model-access/' | relative_url }}">模型接入</a></li>
+    <li><a href="{{ '/docs/zh_CN/build-agent/' | relative_url }}">构建智能体</a></li>
+    <li><a href="{{ '/docs/zh_CN/build-conversational-assistant/' | relative_url }}">构建对话助手</a></li>
+    <li><a href="{{ '/docs/zh_CN/workflow/' | relative_url }}">工作流</a></li>
+    <li><a href="{{ '/docs/zh_CN/knowledge-base/' | relative_url }}">知识库</a></li>
+    <li><a href="{{ '/docs/zh_CN/plugins/' | relative_url }}">插件</a></li>
+    <li><a href="{{ '/docs/zh_CN/forms/' | relative_url }}">表单</a></li>
+    <li><a href="{{ '/docs/zh_CN/application-market/' | relative_url }}">应用市场</a></li>
   </ul>
 </nav>