Update SUS Section of handbook

Why is this change being made?

The SUS section of our handbook currently includes only our Q1 results. We need to update this as a SSOT. This MR adds a new link to the section: https://about.gitlab.com/handbook/engineering/ux/performance-indicators/#perception-of-system-usability.

Does this MR meet the acceptance criteria?

Conformity

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