add zh-CN readme

[Naylor55]

Summary

• Added README.zh-CN.md — a full Chinese (Simplified) translation of the English README
• Added language switch links at the top: [English](README.md) | [日本語版](docs/README.ja.md)

Notes

• All code blocks, badges, tables, and project structure diagrams are preserved as-is
• Internal documentation links (docs/*.md) are kept unchanged; only their display titles are translated
• Technical terms (e.g., VRAM, GGUF, MoE, KV cache) are retained in English where appropriate, with Chinese explanations provided contextually