Figure out a changelog for the OpenAPI schemas

The changelog system currently only cares about the rendered spec text, not any of the other functionality of the repo. We should have a changelog section/area for those.

Start by examining how the existing changelog tracks changes to the rendered spec text. The issue does not name files, tests, or the other functionality to cover, so first determine what repository changes need recording and how a new changelog area should fit. Done means defining and implementing a changelog section for those changes.

Written by the indexing model from the issue text.