Decide on the format and schema of the ASG that will be used by the TCK

The project proposal suggests basing the TCK on an ASG in order to verify that an implementation parses and comprehends AsciiDoc in compliance with the specification. If we decide to adopt that strategy, as discussed in #6, we need to decide on the format and schema for the ASG.

The purpose of this issue is to pick a format for the ASG (e.g., JSON) and map out the general schema of the data. We don't have to think of every possible use case as the schema will continue to evolve over time. But this issue should at least outline the common elements and leave room for it to be elaborated.