Add datatypes to model docs

[rrfreimuth]

Please add specific datatypes to the model docs. The current description gives a good baseline understanding of what each field represents, but to implement the spec more information is needed. Please specify the type of each field, linking to relevant standards where possible (e.g., dateOfBirth could follow ISO 8601 or FHIR R5 date) and fully documenting any value sets that are used (e.g., sex, deceased, affected). When both a coded concept and free text are permitted, it should be clear how a recipient will tell the difference between them. The format of identifiers should be specified if constraints on character set or syntax exist.