Docs Needed: Cube Cloud InstanceURL

Cube Cloud users can provide an InstanceURL for `embeddable:dev` so that they can test between different instances, but it's not really mentioned anywhere in the docs. See this ticket for more info: https://trevorio.atlassian.net/browse/EM-1408?atlOrigin=eyJpIjoiNmI5ZTE5OWUyNWQ2NGE3MTlkNzg3YjUyMGEyYjVlNjkiLCJwIjoiamlyYS1zbGFjay1pbnQifQ