Add example usage for instance-level Duo usage API endpoint to GraphQL documentation

Follow-up to #582153 (closed)\n\nAdd documentation and example usage for the new instance-level API endpoint for retrieving raw Duo usage event data to https://docs.gitlab.com/api/graphql/duo_and_sdlc_trends\n\nThis should include:\n- Example GraphQL query showing how to use the new endpoint\n- Description of the endpoint's capabilities\n- Authorization requirements (admin access)\n- Any relevant parameters and response structure