Update index_en.rst

fix #8956

doc/v2/getstarted/index_en.rst

 GET STARTED
 ============
 
+If you want to quickly know how to use PaddlePaddle, please refer to the following guide:
+
 ..  toctree::
   :maxdepth: 1
 
   quickstart_en.rst
+  
+  
+While using PaddlePaddle to build applications, please understand some basic concepts.
+Here is an example of linear regression. It introduces use flow of PaddlePaddle, including data format, model configuration and training, etc.
+  
+  ..  toctree::
+  :maxdepth: 1
+  
   concepts/use_concepts_en.rst