[SPARK-3143][MLLIB] add tf-idf user guide

Moved TF-IDF before Word2Vec because the former is more basic. I also added a link for Word2Vec. @atalwalkar
What does "distributed" mean in "distributed vector representation"?  Does it refer to the fact that the computation is distributed?  If so, could we say "...computes vector representation of words in a distributed fashion."