Document comprehensive list of test cases for Open Booking API from a end-user perspective

[nishaldesai]

[nishaldesai]

@nickevansuk https://docs.google.com/spreadsheets/d/129h_LdK4QkhV4Yg-Lck8qsIttQBbiq_HTm6pUM1uGy8/edit#gid=0

Aggregated over real world testing with MCRactive, Legend, Providers Portal and Playfootball - amongst others.
Yellow cells need your attention / clarification.

I converted to an issue in the developer-documentation rep as that's what you did for the previous one. Please move if I have put this in the wrong repo.