Create "data dictionary" for all SCTK fields and schema

We need a comprehensive "data dictionary" of all fields that may be present in a ScanCode output file.
The minimum requirement is a list of fields with type (single value, list or ?) and description. This should, of course, be versioned SCTK releases.
We have some of this information for the CSV output at https://scancode-toolkit.readthedocs.io/en/latest/cli-reference/output-format.html#custom-output-format (which may not be current), but we do not have it for the full set of output fields in the JSON output.
There is currently no single file in the codebase (like Django models), but two files seem to contain most of the field definitions:
- ./src/scancode/api.py - non-Package fields
- ./src/packagedcode/models.py - Package fields (see section starting line 361)

A first step should be to investigate whether there are existing automated documentation tools for Python that would help us get started.

This Issue supersedes #112 which is pretty stale at this point.