Add support for collapsible multi-level sidebar

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

I would like to have to collapsible navigation sidebar since I organize my documents by using multiple layers of folders, which is frustrating to browse or lookup the content/page I want.

VitePress only supports the top-level folders to be collapsible in the navigation sidebar currently.

Describe the solution you’d like

The default theme that VitePress provided could support multi-level sidebar to be collapsible since many of the people may also face such kind of the problem while using VitePress.

Describe alternatives you’ve considered

I have tried to override the VPSidebarNav component from default theme that VitePress provided, however, the theme configuration overridding is not supported currently (related issue is still open, ref: #235 ), or I would have to duplicate the default theme as a single repository and publish it as a npm package after the development is done by myself. Anyways, I may think this kind of the feature is general for most of the users, it should be provided by VitePress, nor by users.

Additional context

No response

Validations

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• 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.

[kiaking]

I’ll take that PR over to adjust the design a bit 👍

[brc-dd]

I’m currently working on the code groups feature, will review and try to get #1361 merged within this week. Sorry for the delay 😓