Add Markdown blob embed dashboard, tag and group Markdown dashboards

Adds a dashboard to observe Markdown blob embeds (metrics added in gitlab-org/gitlab!253363 (merged); feature is FF'd), and groups it with the Markdown cache upgrade dashboard (from !10918 (merged)), and adds links to Planner Intelligence's dashboard. Also adds some tests \o/

Preview at https://dashboards.gitlab.net/d/aconnor-verify-markdown-blob-embeds. Preview of the newly-tagged Markdown cache upgrade dashboard at https://dashboards.gitlab.net/d/ak-verify-md-cache-version-upgrades.