Add steps for closing product discovery issue
Note: this MR supercedes the previous handbook update: gitlab-com/www-gitlab-com!13773 (closed).
I've updated the handbook with additional guidance on what must be done before considering a product discovery issue complete. I think some people are already doing these items, but this update intends to make this the formal process.
In particular, updating the product discovery issue description with the findings will prevent the situation where in order to figure out what happened with a product discovery issue you must read the original description and all the comments to replay the course of events in your head in order to understand what the eventual finding was. It will also remain a static snapshot of the decisions at that point in time, vs. the delivery issue which will continue to evolve.
Of course none of this is necessary for situations where no separate product disovery issue was created (which is the preferred approach), but since we do still occasionally use these this is intended to be an improvement to that process.
Feedback is of course welcome!