Resolve "Update "Social response time" KPI definition"

Issue

Closes #4391 (closed)

Part of: https://gitlab.com/gitlab-data/analytics/issues/1241

Checklist

• Move definition off OpMetrics Page source/handbook/finance/operating-metrics/index.html.md
• Add definition to new place in handbook with Metric [KPI](/handbook/ceo/kpis/) Definition format
  • Definition includes info on canonical data source and formula, if applicable
  • Mentions goal, if applicable
  • Links to Periscope dashboard, if applicable
• Update link on KPI Mapping page source/handbook/business-ops/data-team/metrics/index.html.md

Solution

This MR:

• is more explicit in the definition on the community advocacy page https://about.gitlab.com/handbook/marketing/community-relations/community-advocacy/#respond-to-every-community-question-about-gitlab-asked-online
• updates the KPI mapping page
• removes the reference from the operating metrics page