Start OpenAPI spec, for #17

[wetneb]

This is a draft of an OpenAPI spec, for #17.

Remaining things to address:

• the schemas we have are JSON Schemas, but OpenAPI needs "OpenAPI Schema Objects" instead, so we'd need to (ideally automatically) convert our schemas to that
• include expected behaviour on invalid input (Clarify expected behaviour on invalid input #171)
• link to the OpenAPI profile from the HTML specs (or include it verbatim even?)

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