Add mdn_url for CustomElementRegistry built-in element support (mdn#2…

…1758)

* Add mdn_url for CustomElementRegistry built-in element support

This adds an MDN URL for the built-in element support subfeature of the CustomElementRegistry API.  This will help explain what it means, as there is no explanation on the MDN page for `CustomElementRegistry` itself (only in the `define` method).

* Remove locale

api/CustomElementRegistry.json

@@ -39,6 +39,7 @@
       "builtin_element_support": {
         "__compat": {
           "description": "Customized built-in element support",
+          "mdn_url": "https://developer.mozilla.org/docs/Web/API/CustomElementRegistry/define#defining_a_customized_built-in_element",
           "tags": [
             "web-features:custom-elements"
           ],