[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

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
[SPARK-2843][MLLIB] add a section about regularization parameter in ALS

atalwalkar srowen

Author: Xiangrui Meng <meng@databricks.com>

Closes #2064 from mengxr/als-doc and squashes the following commits:

b2e20ab [Xiangrui Meng] introduced -> discussed
98abdd7 [Xiangrui Meng] add reference
339bd08 [Xiangrui Meng] add a section about regularization parameter in ALS