docs: add tutorials for 3 usage scenarios with bilingual support

- Tutorial 1: Use any diffusers model without downloading QuantFunc models (Lighting backend)
- Tutorial 2: Download and use SVDQ pre-quantized models for max speed
- Tutorial 3: Export custom models with LoRA baked in
- Split all tutorials into separate EN/ZH files
- Add screenshot placeholders in assets/
- Update main READMEs with scenario introductions and tutorial links
- Copy workflow reference docs to doc/ directory