* run document_preview when samplecodes be tested

* run document_preview when samplecodes be tested

* sphinx-build symbol link; and build-doc default

* FLUIDDOCDIR typo

* download the required configirations and some other scripts

* install required python packages.

* clone specified branch of docs repo, and if failed, clone the default branch

* clean workspace for docs repo

* use the conf.py imported by https://github.com/PaddlePaddle/docs/pull/4222/

* download and install the boscmd

* Optimaze the code comments.

* specify the pypi index server

* only do doc-build when running in cpu mode

* pull docs pr

git log

paddle_pr_info

* install jq

* force using sphinx-build under py3.7

* using our new domain name for preview

* install python package error

* don't build doc default