docs: add Chinese/English documentation switch

[AaronL725]

Summary

• keep the existing Chinese README and proxy-pool guide as the default documentation
• add README.en.md and docs/proxy-pool.en.md
• add reciprocal 简体中文 / English language switches
• keep English documentation structurally aligned with the Chinese originals
• route English README links to the English proxy-pool guide

Verification

A fresh comparison against main confirmed:

• Chinese content is byte-for-byte unchanged after removing the newly added language-switch line
• heading levels, fenced-code structure, and table row structure match between language pairs
• JSON configuration-key structure matches
• README table-of-contents anchors resolve
• all relative documentation/resource paths resolve
• external resource references are unchanged
• only the four documentation files are changed

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