Describe new labeling scheme for L&R Support issues for Fulfillment

Why is this change being made?

Licensing & Renewals Support is collaborating with the Fulfillment Product Management Team to manage a list of issues and epics that Support wants engineering to address in the product. As part of managing that list, we need a well-defined and consistent scheme for labeling the items on the list, and have never had one.

This MR updates the handbook to describe our newly-created scheme. See gitlab-com/support/licensing-subscription/fulfillment-requests#7 (closed) for more information.

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