docs: Rewrite PEFT integration guide with comprehensive examples

Resolves #4376

- Add detailed examples for SFT, DPO, and GRPO trainers with PEFT
- Add QLoRA section with 4-bit and 8-bit quantization examples
- Add Prompt Tuning section with configuration examples
- Update all examples to match current TRL API (LoRA r=32, alpha=16)
- Remove outdated PPO-only focus
- Add troubleshooting section and multi-GPU training guidance

Author: behroozazarkhalili