Update Metrics Dictionary Guide page based on initial review
This page describes product documentation work that was identified during a documentation review of documentation pages associated with the Growth:Product Intelligence stage/group.
- Needs an introductory line that at least links back to the main Usage Ping page.
- This, and other pages, are missing from the left nav. Should be under https://docs.gitlab.com/ee/development/usage_ping/
- Missing from nav: gitlab-docs!1577 (merged)