### What changes were proposed in this pull request?
This proposes to port old PySpark migration guide to new PySpark docs.
### Why are the changes needed?
Better documentation.
### Does this PR introduce _any_ user-facing change?
No. Documentation only.
### How was this patch tested?
Generated document locally.
<img width="1521" alt="Screen Shot 2020-08-07 at 1 53 20 PM" src="https://user-images.githubusercontent.com/68855/89687618-672e7700-d8b5-11ea-8f29-67a9ab271fa8.png">
Closes#29385 from viirya/SPARK-32191.
Authored-by: Liang-Chi Hsieh <viirya@gmail.com>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>
