Make the production readiness issue template more clear

As per the Slack discussion (https://gitlab.slack.com/archives/C014DSVPM44/p1635946191095300?thread_ts=1635944523.094900&cid=C014DSVPM44 and following) we've found that the issue template is not enough clear. It's easy to forget the handbook description of production readiness - which requires the document to be prepared and added to the project with a merge request - and to focus on filling up the issue questions themselves.

This should make it more clear to the DRI that creating the issue is not the end goal but just the beginning of the process 🙂