[Export docs] Move docs to platform folders.

platform/web/detect.py

@@ -48,6 +48,16 @@ def get_opts():
     ]
 
 
+def get_doc_classes():
+    return [
+        "EditorExportPlatformWeb",
+    ]
+
+
+def get_doc_path():
+    return "doc_classes"
+
+
 def get_flags():
     return [
         ("arch", "wasm32"),