[WIP] Add ability to add subcategories in sidebars and update dark mode toggle

[Copilot]

• Update Sidebar.js to support subcategories (nested items with collapsible behavior and proper indentation)
• Update sidebar.json to add a subcategory example (group Components into subcategories)
• Update DarkModeToggle.js to use sun/moon icons from lucide-react with smooth animation
• Verify the changes visually (screenshot)

Original prompt

add the ability to add subcategories in sidebars if not done so yet (and make it not feel like a complete mess). and also make it so that the dark mode toggle is a sun/moon icon instead of a button (with border/bg) with the text in it. and make it well animated

Created from VS Code.

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