This PR improves the `THUDM/LongBench` dataset card by:
- Adding `library_name: datasets` to the metadata for better discoverability and standard Hugging Face Hub practices.
- Introducing a "Used In / Related Work" section that links to the [AttnTrace paper](https://huggingface.co/papers/2508.03793) and its [Hugging Face Spaces demo](https://huggingface.co/spaces/SecureLLMSys/AttnTrace). This highlights an important application of the `LongBench` dataset in recent research on context traceback for LLMs.
- Updating the Arxiv link for the original `LongBench` paper from `.pdf` to `.abs` for a canonical URL.
