Create first draft of OpenAPI specification for website

[david-a-wheeler]

I've wanted to have an OpenAPI specification for the website for a while. Here's an early cut, drafted in part by OpenAPI.

[andrewfader]

Seems good, I'm a fan of auto-generating these

[david-a-wheeler]

To be fair, the API spec is not auto-generated, this one is hand-generated by OpenAPI. We could do that instead, but that'd require some real work.