Resolve "Deprecate wider and narrow MR terminology for simplification"

Why is this change being made?

The current Overall, Narrow, Wider MR Rate taxonomy is confusing. This MR changes MR Rates to be Narrow MR Rates for the Engineering Division.

From #11134 (comment 537148208)

• We can remove the MR rate explanation page - !78269 (diffs) moved to !78269 (diffs)
• We can remove all "Overall" or "Wider" MR Rate PIs - This only updates engineering_function PIs in !78269 (diffs)
• We can delete the Key word "Narrow" since there is nothing to disambiguate Narrow from, the only ones remaining will be Narrow ones from PIs e.g. so "Engineering Division Narrow MR Rate" becomes "Engineering Division MR Rate" - !78269 (diffs)
• We need to promote the "Engineering Division MR Rate" from RPI to KPI - !78269 (diffs)
• We need to promote the "Percent of MRs from Community" from RPI to KPI - !78269 (diffs)

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
• Assign this change to the correct 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 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 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. (this requirement has been removed pending identification of a new DRI for the handbook)

Relates to #11134 (closed)