docs(readme): add Simplified Chinese README

[ShizengLiu]

Summary

This PR adds Simplified Chinese documentation for the localization support introduced in #73.

Changes

• Add a complete Simplified Chinese README (README.zh-CN.md) based on the existing English and French documentation.
• Add Simplified Chinese links to the language navigation in README.md and README.fr.md.
• Update the English and French feature descriptions from bilingual to multilingual.
• Document the project features, integrations, installation options, technical stack, security, and contribution information in Simplified Chinese.
• Reuse the existing project screenshots to keep the documentation consistent across languages.

Validation

• Verified navigation between the English, French, and Simplified Chinese README files.
• Confirmed that this PR contains documentation changes only.