Update docfx.json (SharePoint#3534)

add Docs header

Author: lindalu-MSFT

docs/docfx.json

@@ -34,6 +34,7 @@
     "overwrite": [],
     "externalReference": [],
     "globalMetadata": {
+      "uhfHeaderID": "MSDocsHeader-Dev_SharePoint",
       "extendBreadcrumb": "true",
       "breadcrumb_path":"/sharepoint/dev/breadcrumb/toc.json",
       "ms.suite": "office365",
@@ -60,4 +61,4 @@
     "template": [],
     "dest": "sharepoint-developer"
   }
-}
+}