docs: details about input files and dataset description location

@eguiraud suggested that it would be good to list the number of events in the dataset section. Right now that only is included in the notebook. Additionally, the dataset description https://agc.readthedocs.io/en/latest/versionsdescription.html#datasets currently lives in the version description page, which might be a bit misleading. Perhaps this can be refactored a bit.