* Simply modify beginners_guide/basics/image_classification/index.md
to have the $math$ format instead of `$math$`

* Update the rest of the documentation to move away from `$$` format

* delete video

* delete video link of recognize_digits

* delete video link of image_classification

* delete video link of label_semantic_roles

* delete vedio link of machine_translation

* delete video link of recommender_system

* delete video link of understand_sentiment

* delete video link of word2vec