Fix the doc index for train demo (#3878)

* test=develop,test=document

* test=develop, test=document_fix

* test=develop, test=document_fix

docs/index.rst

@@ -60,6 +60,13 @@ Welcome to Paddle-Lite's documentation!
   demo_guides/rockchip_npu
   demo_guides/mediatek_apu
 
+.. toctree::
+  :maxdepth: 1
+  :caption: 训练示例（预览）
+  :name: sec-train_demo_guides
+
+  demo_guides/cpp_train_demo
+
 .. toctree::
   :maxdepth: 1
   :caption: API文档