[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 
Shall we make the input path configurable and show users the command to run the example code? For example,

~~~
bin/spark-submit --driver-memory 4g examples/src/main/python/mllib/word2vec.py text8_lines
~~~