Update index_en.rst (#9321)

* Update index_en.rst

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* Update index_en.rst

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doc/v2/dev/index_en.rst

 Development
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+PaddlePaddle adheres to the following three sections of code and document specifications.
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+PaddlePaddle uses git for version control and Docker is used for building and testing environment. The code includes Cuda, C++, Python, Shell and other programming languages，which comply with Google C++ Style, Pep-8, and the code base includes style checking by an automatic inspection tool. Code comments need to follow the Doxygen specification. The code that does not meet the style requirements will fail to compile. We provide the following guidelines for the use of Git, build tests and code development.
 ..  toctree::
   :maxdepth: 1
 
   contribute_to_paddle_en.md
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+PaddlePaddle is well documented in English and Chinese. We recommend using the English version of the documents and problem description. The design documents focus on problem descriptions, backgrounds, and are followed by solutions. As documents are generated by Sphinx, code comments should comply with the Sphinx documentation standard. We recommend to use the paddlepaddle.org tool to compile and generate and preview documents locally. Please refer to:
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+..  toctree::
+  :maxdepth: 1
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   write_docs_en.rst
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+PaddlePaddle V2 defines new operations by adding new Layers. You can implement various complex layers by combining basic APIs to satisfy most applications. If you want to customize layer, please refer to the following, and welcome to propose patch.
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+..  toctree::
+  :maxdepth: 1
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   new_layer_en.rst