fix: nav收纳样式问题修复

[wanglinfang2014]

What

🤖 Generated by Copilot at b86a1bc

This pull request enhances the menu component and its documentation in the amis-ui package. It fixes some visual bugs in the menu styles and adds an example of nested submenus in the nav.md file.

🤖 Generated by Copilot at b86a1bc

We are the masters of the menu
We shape the nav with our skill
We fix the arrows and the icons
We make it responsive at will

Why

How

🤖 Generated by Copilot at b86a1bc

• Add a child menu item to the responsive collapse mode example in nav.md (link)
• Hide the redundant expand arrow for submenu items in the horizontal collapse mode in _menu.scss (link)
• Fix the alignment issue of menu icons and text in the saas environment by overriding the line-height property in _menu.scss (link)

[github-actions]

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