docs: Add "It Takes Two: Your GRPO Is Secretly DPO" paper to GRPOTrainer

[DhruvvArora]

What does this PR do?

Adds the "It Takes Two: Your GRPO Is Secretly DPO" paper (arXiv: 2510.00977)
to the GRPOTrainer section of the paper index, sorted by publish date (October 2025).

The paper establishes a formal connection between GRPO and DPO, showing that
GRPO's effectiveness stems from an implicit contrastive objective. It introduces
2-GRPO (num_generations=2), which matches 16-GRPO performance at lower training cost —
directly reproducible via GRPOConfig in TRL.

Related to #4374 (Road to v1).

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline, Pull Request section?

Who can review?

@qgallouedec

---

Note

Low Risk
Low risk: documentation-only change adding a new paper reference and example config; no runtime or API behavior is modified.

Overview
Adds a new entry to docs/source/paper_index.md under the GRPOTrainer paper index for "It Takes Two: Your GRPO Is Secretly DPO" (2510.00977), including a brief summary and a reproducible GRPOConfig example highlighting num_generations=2 (2-GRPO).

^{Written by Cursor Bugbot for commit fdb0536. This will update automatically on new commits. Configure here.}

[qgallouedec]

cool!
thanks

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.