fix: load GAIA from Parquet dataset

[ruirui6946]

Summary

• load the current GAIA Parquet snapshot through its dataset configuration instead of the removed GAIA.py loader
• keep the dataset README so Hugging Face can discover the available configurations
• make the import script safe to import and straightforward to unit test
• document GAIA access, import, and evaluation commands

Why

The upstream GAIA dataset migrated to Parquet and no longer ships GAIA.py. The existing importer therefore fails before any examples are processed.

Closes #245

Validation

• ruff check scripts/data/process_gaia.py tests/scripts/test_process_gaia.py
• ruff format --check scripts/data/process_gaia.py tests/scripts/test_process_gaia.py
• pytest -p no:cacheprovider tests/scripts/test_process_gaia.py -q
• mkdocs build --strict