Document Problem responses in API docs

[matthew-white]

Quoting @lognaturel in Slack:

We don’t currently document detailed error codes. It looks like the .1 variants are the only ones documented. Those codes can be really useful for clients and I’d like to use them in pyodk. … my understanding is that we could document 404.1, 404.2, etc separately with API Blueprint because there’s no semantics around HTTP error codes in the spec, the definitions are using standard data structures

The goals of this issue are:

• Document all Problem codes.
• For .1 variants only: Document the structure of the details property.

This won't fully document everything about Problem responses, but it would be a great first step.