docs: Document the HTTP protocol and JSON objects

[BjarniRunar]

Things are settling down, we should be able to create docs that show how to do common Passcrow tasks using either the Python library, or low-level curl commands.

The target audience for this would be people implementing Passcrow libraries in other languages, since the crypto and recovery management is still complex enough that it's probably not great for individual apps/developers to reinvent those wheels themselves.