docs: add Simplified Chinese translation for README

[kuwakongbai-cloud]

📝 Description

This Pull Request adds a high-quality, fully localized Simplified Chinese (zh-CN) translation for the project's documentation (README_zh.md).

I have also updated the main README.md to include a language switching link ([简体中文](README_zh.md) | English) at the very top, allowing Chinese-speaking developers to navigate seamlessly.

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💡 Why this is helpful

• Lowers the Barrier to Entry: Smart agriculture and IoT are thriving fields for students and developers worldwide. Providing a Chinese version helps the vast Chinese-speaking IoT community and university students understand, deploy, and contribute to this amazing project more easily.
• Maintained Integrity: The translation strictly adheres to standardized local IoT engineering terminology (e.g., standard translations for MQTT Broker, Relay Module, Threshold, and Data Structure) while maintaining 100% synchronization with your original Markdown formatting and layout.

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🛠️ Changes

• ➕ Added README_zh.md (Fully translated Simplified Chinese version).
• 📝 Updated README.md (Added the language navigation link at the top).

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Thank you so much for open-sourcing this wonderful SIH project! It is a great reference for IoT cultivation. Looking forward to your review! 🚀

Best regards,
kuwakongbai-cloud