Clean up ingestion model docs -- specifically document attributes better

[evamaxfield]

https://councildataproject.org/cdp-backend/cdp_backend.pipeline.html#cdp_backend.pipeline.ingestion_models.IngestionModel

• each property should be documented
• try to hide the from_json and from_dict methods
• add example for every one
• mark each model as required or optional

[evamaxfield]

Major doc overhaul, use:

"sphinx.ext.autosummary",
"sphinx_autosummary_accessors",

Database Models
---------------

.. currentmodule:: cdp_backend.database.models
.. autosummary::
    :toctree: generated/

    File
    Person