docs: add link to manage metadata section in OAS and API reference (#13102)

www/utils/generated/oas-output/schemas/BaseProductVariant.ts

@@ -128,6 +128,9 @@
  *   metadata:
  *     type: object
  *     description: The variant's metadata, can hold custom key-value pairs.
+ *     externalDocs:
+ *       url: https://docs.medusajs.com/api/store#manage-metadata
+ *       description: Learn how to manage metadata
  * 
 */