Update to validate.mdx to reflect addition of Extra field to Diagnostic

Author: Maed223

website/docs/cli/commands/validate.mdx

@@ -178,6 +178,9 @@ The nested objects in `diagnostics` have the following properties:
     which may be useful in understanding the source of a diagnostic in a
     complex expression. These expression value objects are described below.
 
+- `extra` (object): An optional object that serves as an extension point for additional 
+  machine-readable information about the problem.
+  
 ### Source Position
 
 A source position object, as used in the `range` property of a diagnostic