FY21-Q4 Enablement KR: Correlate PTO impacts to MR rate by overlaying PTO data to narrow MR rate chart => 50%

Add PTO data to the narrow MR rate chart on this page https://about.gitlab.com/handbook/engineering/development/performance-indicators/enablement/

Registered #9491 (closed) to track the progress.

Sub-department & Group levels dependency: https://gitlab.com/gitlab-com/team-member-epics/access-requests/-/issues/6464