Suggested support for two-level sidebars

[jevantang]

Is your feature request related to a problem? Please describe.

抱歉，我英文不好，所以我写中文了。

我是从 VuePress 迁移来的用户，我有一个场景两级侧边栏的场景。

数据库文档，分为「表和字段」、「字典数据」、「默认配置数据」等一级栏目，其中数据表很多，所以「表和字段」侧边栏又对数据表进行了归类，形成了第二级侧边栏，效果如下图：

Describe the solution you'd like

如问题描述，建议 VitePress 支持两级侧边栏，希望能被采纳，谢谢。

Describe alternatives you've considered

No response

Additional context

No response

Validations

• Follow our Code of Conduct
• Read the docs.
• Read the Contributing Guidelines.
• Check that there isn't already an issue that asks for the same feature to avoid creating a duplicate.

[brc-dd]

Hi! This was already added (#851). It'll be there in the next release.