feat(theme/sidebar): collapsible sidebar

[Sepush]

Summary

A shortcut to toggle the sidebar

Related Issue

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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All committers have signed the CLA.

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[Sepush]

rsp-cs.mp4

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[JounQin]

The design is not quite harmonious IMO, and a new issue for discussion should be raised first before PR, and at least it should be an opt-in feature with a new option.

[Sepush]

The design is not quite harmonious IMO, and a new issue for discussion should be raised first before PR, and at least it should be an opt-in feature with a new option.

Currently, this is just a prototype. I'll submit an issue first to discuss the design and get feedback. After that discussion, I'll refine this PR accordingly. For now, I'll mark this PR as a draft.