Add link to Object.seal() in introduction to Object.isSealed() (#32750)

* Add link to Object.seal() in introduction.

> User: "What does `Object.isSealed()` do?"
> Bad answer: "why, it tells you if it's sealed, duh!" 
> Good answer: "Oh, you want the article on `Object.seal()`, here you go."

* actually define Object Seal in the introduction of Object.isSealed()

Co-authored-by: Estelle Weyl <estelle@openwebdocs.org>

* Update index.md

---------

Co-authored-by: Estelle Weyl <estelle@openwebdocs.org>
Co-authored-by: Joshua Chen <sidachen2003@gmail.com>

files/en-us/web/javascript/reference/global_objects/object/issealed/index.md

@@ -7,8 +7,7 @@ browser-compat: javascript.builtins.Object.isSealed
 
 {{JSRef}}
 
-The **`Object.isSealed()`** static method determines if an object is
-sealed.
+The **`Object.isSealed()`** static method determines if an object is [sealed](/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/seal).
 
 {{EmbedInteractiveExample("pages/js/object-issealed.html")}}