docs: add Qwen3 QLoRA Apple Silicon example

[SysCd]

Adds a minimal Qwen3 QLoRA example for Apple Silicon.

Covers:

• Qwen/Qwen3-8B-MLX-4bit LoRA training
• expected train/valid/test dataset layout
• chat-style JSONL example
• generation with adapter
• disabling Qwen3 thinking mode for shorter direct answers

I tested locally that the model downloads, training starts, adapters save, and generation works on Apple Silicon.