fix(package.json): add docfx.json file

The docfx.json file was missing from the package.json causing
the build process to fail. It has now been added.

Author: extendreality

package.json

@@ -38,6 +38,7 @@
         "*.xml",
         "Documentation",
         "Editor",
-        "Runtime"
+        "Runtime",
+        "docfx.json"
     ]
 }