Automatically derive spec docs from Ignition spec docs

Created originally in Butane by @bgilbert: https://github.com/coreos/butane/issues/390

While the Butane docs intentionally diverge from the Ignition docs at many points, they also share significant material. When we update the vendored Ignition to a newer version, we often forget to (manually) sync the spec docs, causing a slow divergence over time. Occasionally this causes spec fields not to be documented.

Consider automatically generating the Butane specs from the Ignition ones. We'd need a way to specify a set of rules for adding and removing fields relative to the Ignition schema, for adjusting the wording of field descriptions, and for converting camel case fields to snake case.

Start by reading the linked Butane issue and comparing the shared Butane and Ignition spec documentation. Define how rules would add or remove fields, adjust descriptions, and convert camel case to snake case; done means the Butane specs can be derived automatically without losing their intentional differences.