metric app has now a disable_dashboard config option

[AlexAndBear]

WHAT Needs to be Documented?

With PR https://github.com/owncloud/metrics/pull/125, a new config option disable_dashboard has been introduced, to disable the Dashboard UI, see README.md

WHERE Does This Need To Be Documented (Link)?

https://doc.owncloud.com/server/next/admin_manual/enterprise/reporting/metrics.html

WHY Should This Change Be Made?

(Optional) What Type Of Content Change Is This?

• New Content Addition
• Old Content Deprecation
• Existing Content Simplification
• Bug Fix to Existing Content

(Optional) Which Manual Does This Relate To?

• Admin Manual
• Developer Manual
• User Manual
• Android
• iOS
• Branded Clients
• Desktop Client
• Other