Add opportunity for CTO and VPP to review milestone plan to product development timeline

Why is this change being made?

Closes issues: #13615 (closed) and #13620 (closed)

Related to: #13631 (closed) and #13585 (closed)

This MR adds a non-blocking opportunity for CTO and VPP to review and provide the Quad with feedback on milestone plans. This is a proposal to support the work being done as part of the Next Prioritization Working Group. This is an iteration and can be adjusted as needed once we see how it goes as we flow through 15. 3 and 15.4

The opportunity for review has been added without shifting any of the dates currently in the pd-timeline, the idea being that this review should seamlessly fold into the existing workflow.

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.

---