VSD Usage overview - Warn if background aggregations are not enabled

What does this MR do and why?

Ensures we display a warning message about usage overview metrics if the value_stream_dashboard_aggregation setting is not enabled.

• If the aggregation is disabled, display a warning and link to documentation
• If there is "no data" and aggregation disabled show - instead of 0 - !148635 (comment 1858388292)

MR acceptance checklist

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Screenshots or screen recordings

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Aggregation disabled - no data	Aggregation enabled - no data	Aggregation enabled - with data	Dashboard without usage overview

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

You will need a GitLab Ultimate license. Enable the :group_analytics_dashboard_dynamic_vsd feature flag

Feature.enable(:group_analytics_dashboard_dynamic_vsd)

1. Create a group
2. Create some projects in the group
3. Create some issues and MRs for one of the projects
4. Visit the Value streams dashboard - Observe the warning message
5. Enable background aggregations for the group
6. Return to the Value streams dashboard

Related to #455786 (closed)