Add information about dedicated qa and maintenance window

For issue gitlab-com/gl-infra/delivery#19557 (closed)

I added information about the dedicated projects' maintenance window, and qa coverage and processes.

Trying to keep it concise to delivery's focus of reducing downtime. Since the maintenance window is mainly created and validated at the switchboard level, it's good to take note of it, but I didn't think too much detail was needed.

Please make suggestions/edits 🙇