[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 
The input to `Word2Vec` should be sentences instead of individual words, though it doesn't affect the implementation. The following command extract 16 words per line.

~~~
grep -o -E '\w+(\W+\w+){0,15}' text8 > text8_lines
~~~