* add async_training.rst

* add cpu_train_best_practice.rst

* clean code

* add doc for large_scale_sparse_feature_training

* add index for distribute train api guide

* follow_comments

* fix_link_format

* fix_link_format

* follow_comments_large_scale

* follow_comments

fix https://github.com/PaddlePaddle/FluidDoc/pull/400#discussion_r237335307