docs: document data_dir in YAML dataset configuration (#2340)
* docs: document data_dir in YAML dataset configuration
Add a new "Data Directory" section to the manual configuration guide
explaining how to use data_dir as an alternative to explicit data_files
paths. Includes examples for directory-based subsets, automatic split
detection, and combining data_dir with data_files.
Fixes huggingface/datasets#6561
* Apply suggestions from code review
Co-authored-by: Quentin Lhoest <42851186+lhoestq@users.noreply.github.com>
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Co-authored-by: 32134178csa <321dsadsaca@protonmail.com>
Co-authored-by: Quentin Lhoest <42851186+lhoestq@users.noreply.github.com>
