Add example initial query in GraphiQL interface

GraphQL has a big initial learning curve so it would be good to have an example initial query that is displayed when you navigate to the GraphiQL page in Case Explorer. The initial query and some help text comments [can be set via the `defaultQuery ` prop](https://github.com/graphql/graphiql/tree/main/packages/graphiql#props) passed to the `<GraphiQL />` component.