Add guide about using discussions in issues

This update is in response to the "Organize discussions better" bullet from the 11.9 retrospective: https://docs.google.com/document/d/1nEkM_7Dj4bT21GJy0Ut3By76FZqCfLBmFQNVThmW2TY/edit#heading=h.epd5v8lvb9y1

@splattael - please have a look at this update. Let me know if you have any feedback.