Update Streaming API docs to match UI docs structure

What does this MR do and why?

As part of #416449 (closed) we restructured the Streaming UI docs to make it easier for users and also allow for scaling to more streaming destinations. This updates the API docs to match.

Review APP: https://main-ee-127429.docs.gitlab-review.app/ee/administration/audit_event_streaming/graphql_api.html

Current: https://docs.gitlab.com/ee/administration/audit_event_streaming/graphql_api.html

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