DAG MVC: Beta Link, Documentation, Feature Flag

What does this MR do?

This MR adds the three ingredients required for the DAG MVC to go live:

• A beta tag, linked to a feedback issue (#220368 (closed))
• Documentation about the DAG visualization
• Defaulting the feature flag to on

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers