docs: unclear how to obtain SWE-bench dataset #1417

A user trying to run SWE-bench Verified rollouts was blocked on obtaining the dataset. The YAML config (swebench_openhands.yaml) references gitlab_identifier with dataset_name: swebench_verified_for_sweagent_and_openhands, but responses_api_agents/swe_agents/README.md doesn't explain how to download it. The user tried ng_download_dataset_from_hf and got a 401.

It's unclear from the docs:

• Where the dataset is hosted and how to obtain it
• Which download command to use (ng_download_dataset_from_hf, ng_download_dataset_from_gitlab, ng_prepare_data, etc.)
• What credentials (if any) are needed

Additional issues in responses_api_agents/swe_agents/README.md:

• Example commands use internal Lustre paths (e.g. /lustre/fsw/portfolios/llmservice/users/...) that are not useful to external users
• SIF image creation is documented only as a link to an external script in the NeMo-Skills repo (nemo_skills/dataset/swe-bench/dump_images.py). The SIF creation workflow should be self-contained within NeMo Gym's own docs — users should not need to go to a separate repo to figure out how to set up container images.