Automatically derive spec docs from Ignition spec docs

[bgilbert]

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.