Add details about UX reviews for groups without designer

Why is this change being made?

We have seen an increase in the number of MR UX Review requests over the last couple of months with the trend continuing into May. To align with our current capacity, we are updating the documentation in the handbook to reflect a modification in how we handle UX Reviews for the foreseeable future. We will no longer be conducting MR reviews for stage groups without a design DRI, unless it is from a community contributor.

The reasons why these are the MR reviews we are no longer supporting:

• We have aligned designers to support areas of the product that have been given the highest prioritization.
• MRs from teams without design support tend to require significantly more time for a variety of reasons such as, missing designs to compare, missing context, unfamiliarity with long-term goals and aligning short-term iterations, and so on.
• This change allows for the volume of MR reviews designers are assigned to ease opening up their capacity for their stage group work.

Author Checklist

• Provided a concise title for this Merge Request (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, and the content is SAFE
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (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 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
  • The when to get approval handbook section explains the workflow in more detail
• 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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