Moved TF-IDF before Word2Vec because the former is more basic. I also added a link for Word2Vec. atalwalkar

Author: Xiangrui Meng <meng@databricks.com>

Closes #2061 from mengxr/tfidf-doc and squashes the following commits:

ca04c70 [Xiangrui Meng] address comments
a5ea4b4 [Xiangrui Meng] add tf-idf user guide