Hi all,

I am trying to write up the main page of PySpark documentation at https://github.com/apache/spark/pull/29320.

While I think the current proposal might be good enough, I would like
to collect more feedback about the contents, structure and image since
this is the entrance page of PySpark documentation.

For example, sharing a reference site is also very welcome. Let me know
if any of you guys have a good idea to share. I plan to leave it open for some
more days.

PS: thanks @Liang-Chi Hsieh and @Sean Owen for taking a look at it quickly.