Error budget dashboard should include explanatory text and SLA queue attribution

(Placeholder)

The Error Budget dashboard, https://dashboards.gitlab.net/d/general-error-budgets/general-feature-category-error-budgets, currently includes apdex scores and error rates for individual queues, but it's not easy to make the link back to the effect this on error budget. We should improve this.

Additionally, the dashboard should include more explanatory text for new users.