Update product handbook to utilize design system to work autonomously

Improve the wording regarding utilizing the design system.

My reason for changing this is because the focus in the handbook should be on how others can work autonomously, not UX being a blocker. If UX is a blocker, we should be working together to figure out why. It should be a conversation that is focused around solving that problem, not a constant in our handbook.

cc @JobV as this updates the verbiage of your original commit 9715bb9e