Add Product Design KPI about MR Reviews

Why is this change being made?

We're adding a Product Design-specific KPI to the UX Department Performance Indicators to learn our current level of involvement in MR Reviews involving user-facing changes.

We currently have MR reviews, including community contributions, listed as our number one priority.

We're hoping to gain insight into the number of MR reviews being requested by Stage and identify possible process improvements that can help us improve the quality of our product and reduce the amount of UX debt entering our product.

Before merging, we need to determine how to measure.

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the "Maintained by" section in on the page being edited.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies.
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files. (this requirement has been removed pending identification of a new DRI for the handbook)