Docs: add Metrics Dashboard Annotations page to nav
gitlab!29368 (merged) will add a new page to the API section, and it needs to be added to the global nav once gitlab!29368 (merged) lands. I've made this MR dependent on it.
cc to @k_cook (who submitted that MR to me) and @dhershkovitch
Edited by 🤖 GitLab Bot 🤖