docs: fix README language switching between English and Chinese pages

[AperturePlus]

Motivation

• Restore practical language switching between the English root README and the separated Chinese documentation so users can jump between pages from the rendered GitHub view.

Description

• Updated the top language links in README.md to point to README.md and doc/README.zh-CN.md for cross-file navigation.
• Added a short note in the English README.md Chinese section indicating that language can be switched via the top links.
• Created doc/README.zh-CN.md and added symmetric top-of-page links ([English](../README.md) | [中文](README.zh-CN.md)) so the Chinese page can return to the English page.
• Kept the Chinese content in doc/README.zh-CN.md and removed the in-page duplicate Chinese content from the root README.

Testing

• Inspected diffs for README.md and doc/README.zh-CN.md to confirm only the intended link and header changes were made.
• Printed the line-numbered contents of both files to validate the new cross-file language links and the updated Chinese section note in README.md.
• Verified that doc/README.zh-CN.md exists and contains the expected Chinese documentation with the symmetric language-switch links.
• Did not modify or run the repository unit test suite as part of this documentation change.

---

Codex Task