[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
It is used in the original paper and the term "distributed" is from http://www.indiana.edu/~clcl/BEAGLE/Jones_Mewhort_PR.pdf . I have trouble understanding "distributed vector representation" as well. I think "distributed" means we map a single word to multiple values ....