Document `base_model` in modelcard metadata (#1936)
* support base_model in modelcard metadata
* Update src/huggingface_hub/repocard_data.py
Co-authored-by: Daniel van Strien <davanstrien@users.noreply.github.com>
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Co-authored-by: Daniel van Strien <davanstrien@users.noreply.github.com>