[SPARK-3838][examples][mllib][python] Word2Vec example in python

This pull request refers to issue: https://issues.apache.org/jira/browse/SPARK-3838

Python example for word2vec
@mengxr 
@davies I just jued the command listed in the comments. 
"grep -o -E '\w+(\W+\w+){0,15}' text8 > text8_lines"
Did you want me to include a bash script?