docs: remove convert appimage documentation from sidebar

Removed the "Convert appimage" documentation link from the sidebar
navigation for version 1.9.x. This change was made because the appimage
conversion feature is no longer supported or maintained in the current
version, making the documentation obsolete.

Influence:
1. Verify the sidebar navigation no longer shows the "Convert appimage"
option
2. Check that all remaining sidebar links are functional
3. Ensure no internal links reference the removed documentation

docs: 从侧边栏移除转换 appimage 文档

从 1.9.x 版本的侧边栏导航中移除了"转换 appimage"文档链接。此更改是因为
appimage 转换功能在当前版本中不再受支持或维护，使得该文档过时。

Influence:
1. 验证侧边栏导航不再显示"转换 appimage"选项
2. 检查所有剩余的侧边栏链接是否正常
3. 确保没有内部链接引用已移除的文档

Author: myml

.vitepress/sidebar/1.9.x.json

@@ -108,10 +108,6 @@
           "text": "运行应用",
           "link": "/guide/ll-builder/run.md"
         },
-        {
-          "text": "转换 appimage",
-          "link": "/guide/ll-builder/convert.md"
-        },
         {
           "text": "导出 layer 文件",
           "link": "/guide/ll-builder/export.md"