Deprecate wider and narrow MR terminology for simplification

Per latest guidance, we are now tracking “community contribution as % of whole” at the CTO level.

The plan is to simplify metrics for ease of digest.

• Deprecate Narrow/Wide prefix. Let’s focus on open source contributions as a whole.
• Simplify (post/pre-fix, etc) taxonomy and have one single MR Rate per Division and Departments.

Task

• remove the MR rate explanation page
• remove all "Overall" or "Wider" MR Rate PIs
• 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"
• promote the "Engineering Division MR Rate" from RPI to KPI
• promote the "Percent of MRs from Community" from RPI to KPI
• @davis_townsend Make https://about.gitlab.com/handbook/engineering/performance-indicators/#engineering-mr-rate 2 year time span
• @meks Rename all Engineering Departments Narrow MR rate
  • Engineering Division !78269 (merged) !79854 (merged)
  • All Engineering Departments !79885 (merged)
• @lmai1 Make another pass at Member MR Rate

cc @edjdev