Move GDNative docs into gdnative folder

modules/gdnative/config.py

@@ -1,8 +1,12 @@
 
-
 def can_build(platform):
     return True
 
-
 def configure(env):
     env.use_ptrcall = True
+
+def get_doc_classes():
+  return ["GDNative", "GDNativeClass", "GDNativeLibrary", "NativeScript"]
+
+def get_doc_path():
+  return "doc_classes"