Add docs about deployment health metrics

What does this MR do and why?

Describe in detail what your merge request does and why.

Content

Add docs about metrics used to determine if production is safe for a deployment to be started.

gitlab-com/gl-infra/delivery#20396 (closed)

Author Check-list

• Has documentation been updated?
• Ensure there is a correct type label (typemaintenance, typebug or typefeature) assigned to this merge request.