[Docstring] Add docstrings to dspy/datasets/dataloader.py

Hi, I'd like to add docstrings to `dspy/datasets/dataloader.py` as part of #8926. The file currently has no docstrings on any of its public methods (from_huggingface, from_csv, from_pandas, from_json, from_parquet, from_rm, sample, train_test_split). I'll follow the Google Python style guide as specified in #8926. Please let me know if this file is available to work on!