docs: add Chinese documentation and language switcher

[DennisYang0611]

• Add complete Chinese README.zh-CN.md with full translation
• Add language switcher links to both English and Chinese README files
• Improve accessibility for Chinese users

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[cla-assistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

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Dennis seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

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