docs(diffusion RL v0.1): add single-prompt multi-generation guide

[MikukuOvO]

Summary

This draft PR adds documentation for the diffusion RL v0.1 branch:

• Adds an advanced guide for single-prompt multi-generation in Miles Diffusion.
• Explains the relationship between prompt groups, n_samples_per_prompt, diffusion_microgroup_size, and SGLang-D num_outputs_per_prompt.
• Documents current OCR and PickScore recipe settings, the SGLang-D dependency, known limitations, validation checklist, and practical guidance.

Validation

• Ran git diff --check rockdu/diffusion_RL_v0.1...rockdu/docs/diffusion_RL_v0.1_fengliny.
• Confirmed the PR-style diff adds docs/advanced/single_prompt_multi_generation.md only.
• Confirmed there was no existing open PR from Rockdu:docs/diffusion_RL_v0.1_fengliny into diffusion_RL_v0.1 before opening this PR.

Notes

This is intentionally opened as a draft so the same PR can continue to receive follow-up documentation updates without creating duplicate PRs.