Description of output fields

[nickmalleson]

Sorry if I have missed something, but I can't find a clear description of the variables that the SPC actually outputs. I suspect this is documented somewhere, but it may be worth linking from the main readme and/or the introductory sections of the manual.

[dabreegster]

https://alan-turing-institute.github.io/uatk-spc/using_use_output.html
From here, we link a few places -- are any of them helpful? If so, we could make them more discoverable:
https://github.com/alan-turing-institute/uatk-spc/blob/main/synthpop.proto
https://alan-turing-institute.github.io/uatk-spc/understanding_data_schema.html