docs(diffusion RL v0.1): add quick start, OCR example, accuracy guide, and roadmap

[zhihengy]

Summary

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

• Adds a quick start guide for environment setup.
• Adds task-level dependency documentation, including OCR reward dependencies.
• Adds a runnable Qwen-Image OCR example under examples.
• Adds an advanced note on rollout parallel accuracy.
• Adds an external-facing diffusion RL roadmap.

Validation

• Ran git diff --check origin/diffusion_RL_v0.1..HEAD.
• Confirmed the local branch is synced with origin/docs/diffusion_RL_v0.1_zhihengy 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.