enhancement: surface required CpG sites in docs for models without a coefficients file

Models like HurdleInflammAge don't have a model.file entry but do expose their required sites via methylation_sites(). The doc generator currently shows "N/A" for these models.

A future improvement could instantiate the model (or extract sites statically) to populate the coefficients column. This would require reworking how doc/generate.py accesses model info since it currently only reads static definition dicts, not instantiated objects.

Ref: https://github.com/bio-learn/biolearn/pull/197#pullrequestreview-2710853685