docs(blueprint/entity-test-helper): Add module description

Author: TheJanzap

blueprint/cli/blueprints/entity-test-helper/file.rs.liquid.liquid

@@ -1,3 +1,6 @@
+//! This entity test helper for [`{{entity_struct_name}}`] allows making specific database access functions available
+//! only for application tests but not for actual application code.
+
 use crate::entities::{{entity_plural_name}}::{{entity_struct_name}};
 use fake::{faker::name::en::*, Dummy};
 use sqlx::postgres::PgPool;