[Export docs] Move docs to platform folders.

platform/windows/detect.py

@@ -192,6 +192,16 @@ def get_opts():
     ]
 
 
+def get_doc_classes():
+    return [
+        "EditorExportPlatformWindows",
+    ]
+
+
+def get_doc_path():
+    return "doc_classes"
+
+
 def get_flags():
     arch = detect_build_env_arch() or detect_arch()