Capture our "Product Structure over Group Capacity" notion in Handbook

Given our hard structure of groups around product categories (which should be externally understandable), it is an anti-pattern to suggest that we move a category or capabilities in a category based on group capacity. Instead we should re-allocate appropriate capacity to appropriate groups.

“If we ever move something from one category to another because of eng or PM capacity, we're doing it wrong.”

Closes Product#65 (closed)