Note customizable VSA only at group level

What does this MR do?

Previously the documentation for customizable Value Stream Analytics did not explain that it is only available at the group-level feature, so this adds a note to the docs.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content