clean todo in cluster_howto

* windows doc

* fix comment. test=develop

* create how to write docs

* Update write_docs_cn.md

* Update write_docs_cn.md

* Update write_docs_cn.md

* Update fluid_design_idea.md

* Update install_Ubuntu.md

* Update install_MacOS.md

* Update install_CentOS.md

* Update install_Windows.md

* Update compile_CentOS.md

* Update compile_MacOS.md

* Update compile_Ubuntu.md

add doc for distribute lookup table

mv sequence.rst to correct path

update submodules

refine detection.rst

update inference.rst

add detection doc in api_guide 

* add metrics api guide

* update metrics.rst

* update metrics.rst

* update metrics.rst

* update metrics.rst

* update metrics.rst

* update metrics.rst

API guide to sequences

reorder the api

Add models' saver and reader.

update build_and_install_lib_cn

mv pooling.rst to the correct path

Add pool api guide 

Add api guide for math API.

Add activation doc