Consider generating spec docs from a machine-readable description

[bgilbert]

It could be useful to have a machine-readable analog to the spec docs: either one per spec version, or perhaps a single file that specifies the versions in which each config field is available. This might help with coreos/butane#390, and might also help reduce maintenance burden.

The ideal place to store this information is in the JSON Schema, but that might require an infeasible amount of tooling. Also, the docs' understanding of which fields are required doesn't always match the schema, since the docs describe higher-level semantics.