This generates api docs for api types when we do all the other codegen.
Doing this here rather than docs is part of trying to simplify the docs
release process, and will remove the need to manually generate these
when releasing docs, which has been a source of pain in the docs WG.
As an additional bonus, this means you can view API docs for a
particular eventing version or commit in the github UI by going to
docs/eventing-api.md, which is nice.
Julian Friedman