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 gemma.cpp.

Key Changes

• Added README.zh.md covering:
  • Architecture and target audience (minimalist C++ runtime for research & experimentation, Highway SIMD acceleration)
  • Features overview (pure CPU inference for Gemma 2-3 & PaliGemma 2, GEMM mixed-precision with autotuning, custom fp8/sfp weight compression, thread pool, streaming C++ APIs)
  • Quick start workflows (CMake on Unix/WSL/Windows, Bazel, Make)
  • Weight acquisition (Kaggle / Hugging Face Hub) and extraction
  • PaliGemma 2 vision-language model usage
  • T5Gemma encoder-decoder model conversion & inference
  • Single-file SBS weight format migration
  • Performance optimization guidelines & FAQ
  • Library integration (FetchContent and building libgemma)
• 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.
• Targeted the dev branch as requested in project contribution guidelines.

No code or core project configuration files were altered.

[jan-wassenberg]

Thanks for adding :)