dev/ci: generate runtype reference docs with changed.None

Created by: bobheadxi

Let generated reference docs be a subset of steps that are guaranteed to be in the pipeline, rather than a superset, which can be surprising.

In the future we might want to be more clever about this to generate more accurate docs for runtypes that run conditional steps, but hopefully this behaviour is more predictable for now.

See thread: https://sourcegraph.slack.com/archives/C01N83PS4TU/p1647642900757389

Test plan

tests pass, manual review of generated docs