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