Restructure Product Designer MR reviews guidelines

Why is this change being made?

This MR depends on gitlab-org/gitlab!70930 (merged) being merged.

This MR is related to the FY22-Q3 KR: Improve the quality of design MR reviews, and aims to provide more structure and trim down the content in the Product Designer MR reviews guidelines

• Adds workload, preview, review and handoff info.
• Updates content in the Product Designer handbook to link the new design and UI changes checklist added in gitlab-org/gitlab!70930 (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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