docs: set English as explicit default language to fix locale auto-redirect (CON-49)

Mintlify auto-detects browser locale and routes users to matching
language pages. Users with Arabic browser settings were being
redirected to /ar/ pages unexpectedly.

Setting 'default: true' on the English language entry ensures
English is always the default, regardless of browser locale.
Arabic translations are preserved and accessible via the
language switcher.

docs/docs.json

@@ -3339,7 +3339,8 @@
               }
             ]
           }
-        ]
+        ],
+        "default": true
       },
       {
         "language": "pt-BR",