Example of a dataset with nested fields. (#745)

Example describing the `conversations` record set of the
HannahRoseKirk/prism-alignment dataset from HF:
https://huggingface.co/datasets/HannahRoseKirk/prism-alignment

Also added output of running:

`python load.py --jsonld
../../../../datasets/1.0/huggingface-prism-alignment/metadata.json
--record_set conversations --debug --num_records 1 --update_output`