docs: add readonly prop to component props definition

Author: ivictbor

adminforth/documentation/docs/tutorial/03-Customization/02-customFieldRendering.md

@@ -194,6 +194,7 @@ const props = defineProps<{
   meta: any;
   resource: AdminForthResourceCommon;
   adminUser: AdminUser;
+  readonly: boolean;
 }>();
 
 const emit = defineEmits(["update:value"]);