[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 
Those should be complete sentences: `This example uses text8 file from ... .`, `Unzip the file and split it into lines using ...`, `This was done ... in local mode.`