docs: add Chinese documentation and restructure docs directory

[Tsan1024]

MR Description

Background

The project currently lacks a structured Chinese documentation, which makes it harder for Chinese-speaking developers to understand and adopt MLX-VLM.

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Changes

This MR introduces Chinese documentation and reorganizes the existing docs structure:

• Added README_CN.md as the Chinese introduction
• Moved existing English documentation to docs/en/
• Created a new docs/cn/ directory for Chinese documentation
• Added corresponding Chinese docs

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Motivation

• Improve accessibility for Chinese-speaking users
• Enhance documentation clarity and organization
• Facilitate broader community adoption

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Impact

• No code changes
• No impact on existing functionality
• Documentation-only update

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Notes

Further improvements may include:

• Chinese quick start guide
• FAQ section
• More detailed bilingual documentation

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Checklist

• Documentation structure is clear
• Changes are limited to docs only
• No impact on existing features