checkpoint doc

fix redundant env var on ps

This reverts commit 138401b7.

Fix Cross Ref

* first draft(part)

* Update index.rst

* Delete build_from_source_cn.rst

* Delete docker_install_cn.rst

* Delete index.rst

* Delete pip_install_cn.rst

* Delete foo.rst

* Delete foo.rst

* Delete overview.rst

* Delete paddleci.png

* Create foo.rst

* add install_doc

* Delete foo.rst

* delete surplus lines

* fix pip install system type and typo

* add windows package download address

* add .zip and fix path of paddle_build.sh

* fix paddle_build.sh

* Update install_doc.rst

* fix ref links

* Update install_doc.rst

add cluster quickstart

Feature/single node training

Merge branch 'develop' of https://github.com/PaddlePaddle/FluidDoc into feature/single_node_training

Add doc for simple model configuration.

add doc for save_and_load

Prepare Data