### What changes were proposed in this pull request?
This PR proposes to restructure API files according to the layout, see https://github.com/apache/spark/pull/32799. Now the pandas APIs on Spark are under a separate directory which is same level as other modules such as Spark SQL.
```bash
tree reference
```
**Before:**
```
reference
├── index.rst
├── ps_extensions.rst
├── ps_frame.rst
├── ps_general_functions.rst
├── ps_groupby.rst
├── ps_indexing.rst
├── ps_io.rst
├── ps_ml.rst
├── ps_series.rst
├── ps_window.rst
├── pyspark.ml.rst
├── pyspark.mllib.rst
├── pyspark.pandas.rst
├── pyspark.resource.rst
├── pyspark.rst
├── pyspark.sql.rst
├── pyspark.ss.rst
└── pyspark.streaming.rst
```
**After:**
```
reference
├── index.rst
├── pyspark.ml.rst
├── pyspark.mllib.rst
├── pyspark.pandas
│ ├── extensions.rst
│ ├── frame.rst
│ ├── general_functions.rst
│ ├── groupby.rst
│ ├── index.rst
│ ├── indexing.rst
│ ├── io.rst
│ ├── ml.rst
│ ├── series.rst
│ └── window.rst
├── pyspark.resource.rst
├── pyspark.rst
├── pyspark.sql.rst
├── pyspark.ss.rst
└── pyspark.streaming.rst
```
### Why are the changes needed?
To make the directory structure easier to follow.
### Does this PR introduce _any_ user-facing change?
No.
### How was this patch tested?
Manually built and tested the docs.
Closes#32812 from HyukjinKwon/SPARK-35646-followup.
Authored-by: Hyukjin Kwon <gurwls223@apache.org>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>
### What changes were proposed in this pull request?
This PR proposes to change from:
![Screen Shot 2021-06-07 at 1 40 47 PM](https://user-images.githubusercontent.com/6477701/120960027-fc302400-c795-11eb-96fb-73ac1d8277fe.png)
to:
![Screen Shot 2021-06-07 at 1 41 19 PM](https://user-images.githubusercontent.com/6477701/120960074-0fdb8a80-c796-11eb-87ec-69a30692fdfe.png)
### Why are the changes needed?
pandas APIs on Spark (pandas on Spark) is a package in PySpark in the end. So it has to be documented in the same level with other packages (e.g., Spark SQL).
### Does this PR introduce _any_ user-facing change?
Yes, it changes the structure of the docs. To end users, no as it's only in development branch.
### How was this patch tested?
Manually tested as above.
Closes#32799 from HyukjinKwon/SPARK-35646.
Authored-by: Hyukjin Kwon <gurwls223@apache.org>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>