Update UX MR review handbook content

Why is this change being made?

We are updating how we make MR review assignments to Product Designers. Included in this MR is an update to the UX handbook to describe how we will be handling stage group MRs and community contributions.

There are a number of benefits to this change such as:

• Evenly distribute MR Reviews across Product Designers in the UX Department. The current disparity ranges from some ICs averaging 24+ MR reviews a milestone to others averaging 1 or fewer.
• Create awareness of the end-to-end GitLab product by having designers involved in reviewing work outside of their normal areas. This can later lead to holistic solutions and seeing connections between stage groups (and Pajamas) that would otherwise get missed.
• Potentially improve communication and documentation of design solutions as we're now relying on another designer to confirm a solution was developed as intended.
• Designers will get visibility and experience in other areas of the product, potentially discovering a new passion or area of interest to work.

Related: gitlab-org/gitlab!84491 (merged)

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
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added.
• Assign reviewers for this change to the correct DRI(s)
  • 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 affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR.
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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