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

[JasonYeYuhe]

Description

This pull request adds a comprehensive Simplified Chinese translation of the documentation (README.zh.md) for FastChat.

Key Changes

• Added README.zh.md covering:
  • Overview of FastChat & Chatbot Arena (10M+ chat requests, 1.5M+ human votes)
  • Installation (pip & source on Linux / Mac)
  • Model weights (Vicuna v1.5, LongChat v1.5, FastChat-T5)
  • CLI inference (Single GPU, Multi-GPU, CPU, Apple Silicon Metal, Intel XPU, Ascend NPU, 8-bit quantization & CPU offloading)
  • ModelScope integration
  • Distributed serving architecture & Web GUI (controller, model workers, Gradio server)
  • Chatbot Arena local deployment (multi-model battle UI, API configurations for OpenAI, Anthropic, Gemini)
  • OpenAI-compatible RESTful API & SDK
  • MT-Bench LLM-as-a-judge evaluation
  • Local GPU & SkyPilot fine-tuning pipelines
• Added bidirectional language switchers (English · 简体中文) in both README.md and README.zh.md.
• Added a volunteer maintenance note committing to long-term synchronization with upstream documentation updates.

No code or core project configuration files were altered.

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