Add rollout vs backfill view to Markdown cache dashboard

Builds on top of Add Markdown cache version upgrades dashboard (!10918 - merged) — we added the kind label in Markdown cache upgrade counter clarifications (gitlab-org/gitlab!243166 - merged), so we can determine how much of the upgrades happening are actually part of a rollout, and how much would be happening anyway (stale caches even without the active rollout).

Verified with test-dashboard.sh:

(above monolith MR only just hit canary, hence the slow start on the by-kind graph. I'm the only one looking at this dashboard so we're good to merge whenever.)