I imagine this can be improved in various ways, but I've initialized
this with all the **Changes** entries recorded in either zulip.yaml or
the rest of the API documentation, and I expect we'll be able to
iterate on this effectively.
It'll also be useful as a record of changes that we should remember to
document the API documentation as we document more endpoints that
currently don't discuss these issues.
While working on this, I fixed various issues where feature levels
could be mentioned or endpoints didn't properly document changes.
cbe0723199