Add Group relations export API docs

What does this MR do?

A follow up from #328216 (closed) to add documentation for newly added export API endpoints.

Mentions #328372 (closed)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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• Documentation (if required)
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• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

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

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