Update new MR rate page to simplify content and make it reflect the two types of KPIs that relate to it

Why is this change being made?

This MR simplifies our MR rate content and make it reflect 1.) the family of department member mr rate KPIs, and the big KPI that includes all of R&D and open source contributions.

Review app:

• https://update-new-mr-rate-page.about.gitlab-review.app/handbook/engineering/merge-request-rate/

This MR merges into a feature branch in order to give us a sane diff. Once that happens we need to remove Draft: from this MR !58195 (diffs) which moves the content from a sub-section to a dedicated page, and merge it to get it into production.

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][transparency]
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to your manager.
  • 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][mr-buddies-slack].
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files.