Setting the ground work to eliminate metrics pages

This MR is the first step to moving away from having a GitLab Metrics page. https://gitlab.com/gitlab-data/analytics/issues/1241

This MR does the following:

• adds a large warning to the existing operating metrics page that that is not how we work any more.
• reorganizes the metrics by function. This is temporary but will make it easier to move the metrics to the appropriate function-specific pages in the very near future.
• creates a new page under the data team's section of the handbook where we can crosslink a term and its definition. This is meant to make it easier to find terms during this transition period (but I can remove this if it seems like the opposite of what we're aiming for).
• updates CODEOWNERS to reflect when the data team should be update and gives me (personally) additional ability to monitor the Operating Metrics page

Next steps:

• start moving metrics (Chase has volunteered to prioritize finance) out of the Operating Metrics page into the appropriate place in the handbook.