UX handbook update

Updating the UX Docs to reflect the updated assignment process discussed with @jschatz1 @mydigitalself and others in a recent process meeting.

All issues marked deliverable are to be assigned at the start of the milestone. In addition, all parties assigned will remain assigned until the issue is closed. The goal is to have eyes on issues from day one, and to keep the relevant parties assigned to answer questions that may come up and to do reviews of MR's.