Add documentation for panel types

What does this MR do?

This adds documentation on how to add area and single stat panel types to our monitoring dashboard using the custom dashboard feature

Does this MR meet the acceptance criteria?

Conformity

• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides

Related to: https://gitlab.com/gitlab-org/gitlab-ce/issues/60008 gitlab-org/monitor/general#16 (closed)