Update the paper index

Any PR that implements a method, algorithm, or training approach from a research paper must add a corresponding subsection to docs/source/paper_index.md. When reviewing such a PR, check that the file was updated.

Entry format

The file is organized as one ## section per method family (usually one per trainer), each holding ### subsections, one per paper.

Each entry contains:

1. The paper title as the ### heading.
2. A paper link line, using the Hugging Face paper page (same ID as arXiv), never an arxiv.org link:

   **📜 Paper**: https://huggingface.co/papers/<id>
   

3. A few sentences on what the paper introduces and how it maps to TRL.
4. A Python snippet showing the TRL config that reproduces the paper's setting, with the paper's hyperparameters quoted in comments:

   from trl import GRPOConfig, GRPOTrainer