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

[JasonYeYuhe]

What does this PR do?

This PR introduces a comprehensive, high-quality Simplified Chinese translation of the README.md (README.zh.md) to help Chinese-speaking developers, researchers, and engineers learn, adopt, and deploy LitGPT.

Key Changes

• 1:1 Parity Documentation (README.zh.md): Complete translation of the entire README.md, including:
  • 20+ supported open LLMs (Llama 3.x, Qwen2.5/Qwen3, Phi-4, DeepSeek-R1 Distill, Gemma 2/3, Mistral/Mixtral, etc.)
  • End-to-end workflows (CLI quickstart, finetuning with LoRA/QLoRA, serving/deploying, evaluation, chat/inference, pretraining, continued pretraining)
  • State-of-the-art features (Flash Attention v2, FSDP, TPU/XLA, quantization, low-precision)
  • YAML training recipe configuration guide and command-line overrides
  • Project highlights (SAMBA, NeurIPS 2023 Challenge, TinyLlama, MicroLlama) and tutorials index
• Bidirectional Language Navigation: Added quick switcher between English and Simplified Chinese at the top of both README.md and README.zh.md.
• Volunteer Maintenance: Includes standard maintenance notice indicating community volunteer maintenance by @JasonYeYuhe.

Checklist

• Commits are signed off (-s)
• Markdown formatting, tables, image badges, and links have been verified for integrity
• No code or build system modifications

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