docs: add Simplified Chinese translation (README.zh.md)

[JasonYeYuhe]

Description

This Pull Request introduces the official Simplified Chinese translation of the README (README.zh.md) for MiroThinker.

Motivation
MiroThinker is a state-of-the-art open-source research agent that has shown exceptional performance on both English and Chinese benchmarks (like BrowseComp-ZH). Providing a high-quality, manually verified Chinese README will help Chinese developers and researchers better understand the project's architecture, key features, and evaluation results, fostering more contributions from the Chinese open-source community.

Changes Made

• Created README.zh.md with a full, high-quality translation of the latest README.md.
• Added a language switcher at the top of both README.md and README.zh.md for seamless navigation.
• Ensured strict structural parity with the original document, including all tables, benchmark results, and configuration guides.
• Accurately translated technical terms (e.g., "Interactive Scaling", "Trace Collection", "Benchmark Evaluation").

Validation

• Verified markdown syntax and link integrity.
• Manually checked parity against the current main branch.

Thank you for building such a powerful and open research framework! I'm excited to help more users discover MiroThinker.