[next] Add Collapsable feature to SidebarGroup

[hyp530]

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

User experience is bad when I have too many SidebarGroups. The Collapsable feature of SidebarGroup (similar to VuePress@Next) would be very necessary, which should be quite commonly required.

Describe the solution you'd like

I understand VitePress might intend to provide a mininal default theme, rather than create another vuepress. But still, I would strongly recommend you to add this feature.

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.

[kiaking]

Thanks for bringing it up! Yes, this is included in the plan 👍

[kiaking]

Done!