Created DataFields.md as reference for data fields

[Wogiebear]

This page should contain pretty much a raw dump of the corresponding table in the database.
TO DO: retrospectively add a plain english description field to the database tables so that the 'Details' comments can be kept at source.

[Wogiebear]

@marjaimate @sciritai
I'm proposing this extra reference doc. The idea would be that we'd just dump the results of a select from the corresponding table in the database. That way, the living database becomes the living doc as well.
I plan to reference this page from the Api.md because the app developers should know what fields to expect in each response, what type for each field and all possible values for each field.