Fix some style issues in our Sphinx extension, as reported by flake8.

Add directives to our custom-domain extension to declare objects and
attributes, and use these to specify the schema. In addition to
providing a slightly more structured way to specify attribute
parameters, this also makes it possible to create a data model of the
schema which we will eventually use to generate a JSON schema that can
be used to partially validate CPS JSON.

Using the structured information extracted from the documentation (see
previous commit), build and write a JSON schema. This also introduces a
new utility library to convert build the schema description from the set
of objects and attributes.