Sets the run's PDF summary as its workflow version's example
(docs/run-examples.md). Needs edit permission on the workflow, read
access to every dataset the run wrote to, and a SUCCEEDED run.

• A READY PDF for this run (refresh false): 200 with it, and it is
  (re)set as the version's example; nothing is generated or counted.
• A job already in flight for this run: 202 with it.
• Otherwise a new job starts (202): it takes the workflow's lock and
  counts against the daily per-workflow, per-user and per-workspace
  limits; refresh true regenerates a PDF that exists.

503 when examples are not enabled.