docs: add Chinese-specific examples to README.cn.md

[ehongon]

Description

This PR adds a new section "中文使用示例" (Chinese Usage Examples) to the Chinese README (README.cn.md). These examples are specifically tailored for Chinese developers to better understand how to use Rich with Chinese content.

Changes

Added the following examples:

1. 中文表格示例 - Demonstrates rendering tables with Chinese characters
2. 中文进度条示例 - Shows progress bars with Chinese descriptions
3. 中文 Markdown 渲染 - Example of rendering Chinese Markdown content
4. 中文文本样式 - Various text styling examples in Chinese
5. 中文树形结构 - Tree structure visualization with Chinese labels

Why This Change?

• The existing Chinese README is a direct translation but lacks examples specific to Chinese use cases
• Chinese developers often need to handle Chinese characters in their CLI applications
• These examples help bridge the gap between the English documentation and Chinese user needs

Checklist

• Added practical Chinese-specific examples
• Code examples are tested and working
• Documentation follows the existing style
• No breaking changes