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
   
   training_args = GRPOConfig(
       beta=0.001,  # "the KL coefficient to 0.001"
       num_generations=16,  # "For each question, we sample 16 outputs..."
   )
   

   When the paper doesn't specify hyperparameters, say so in a comment rather than inventing values.

Placement

• If the paper belongs to an existing method family, add it under that ## section.
• If it introduces a new trainer or family, add a new ## section with a one-line pointer to the trainer, e.g. Papers relating to the [GRPOTrainer].