UX Growth Handbook Updates

Why is this change being made?

Updating the UX Growth strategy page to include Capacity Template and section hierarchy. Follow up of our retrospective gitlab-org/gitlab-design#1052 (closed)

Does this MR meet the acceptance criteria?

Assign to DRI

• Did you assign this change to the correct DRI of the page or information you are changing?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what