### What changes were proposed in this pull request?
This PR proposes to write the main page of PySpark documentation. The base work is finished at https://github.com/apache/spark/pull/29188.
### Why are the changes needed?
For better usability and readability in PySpark documentation.
### Does this PR introduce _any_ user-facing change?
Yes, it creates a new main page as below:
![Screen Shot 2020-07-31 at 10 02 44 PM](https://user-images.githubusercontent.com/6477701/89037618-d2d68880-d379-11ea-9a44-562f2aa0e3fd.png)
### How was this patch tested?
Manually built the PySpark documentation.
```bash
cd python
make clean html
```
Closes#29320 from HyukjinKwon/SPARK-32507.
Authored-by: HyukjinKwon <gurwls223@apache.org>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>