Add clarification that top navigation requires UX maintainer approval

Why is this change being made?

There have been some situations where a change to the main navigation is debated in the merge request. As these are high-impact areas, a deep debate and discussion/research is needed before allowing any change in those areas. This MR adds that guidance to the handbook.

Additional context can be found at https://docs.google.com/document/d/1KObJ3f-NwR1aMTM2qSbGN__tf9atldLD300bE_8YYJc/edit Related merge request that needed to be closed and where this guidance would have been helpful to have in advance is gitlab-org/gitlab!104244 (comment 1181703146)

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.
• 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
• 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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