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