[SPARK-32507][DOCS][PYTHON] Add main page for PySpark documentation

### 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>

python/docs/source/index.rst

@@ -21,8 +21,44 @@
 PySpark Documentation
 =====================
 
+.. TODO(SPARK-32204): Add Binder integration at Live Notebook.
+
+PySpark is an interface for Apache Spark in Python. It not only allows you to write
+Spark applications using Python APIs, but also provides the PySpark shell for
+interactively analyzing your data in a distributed environment. PySpark supports most
+of Spark's features such as Spark SQL, DataFrame, Streaming, MLlib
+(Machine Learning) and Spark Core.
+
+.. image:: ../../../docs/img/pyspark-components.png
+  :alt: PySpark Compoenents
+
+**Spark SQL and DataFrame**
+
+Spark SQL is a Spark module for structured data processing. It provides
+a programming abstraction called DataFrame and can also act as distributed
+SQL query engine.
+
+**Streaming**
+
+Running on top of Spark, the streaming feature in Apache Spark enables powerful
+interactive and analytical applications across both streaming and historical data,
+while inheriting Spark’s ease of use and fault tolerance characteristics.
+
+**MLlib**
+
+Built on top of Spark, MLlib is a scalable machine learning library that provides
+a uniform set of high-level APIs that help users create and tune practical machine
+learning pipelines.
+
+**Spark Core**
+
+Spark Core is the underlying general execution engine for the Spark platform that all
+other functionality is built on top of. It provides an RDD (Resilient Distributed Dataset)
+and in-memory computing capabilities.
+
 .. toctree::
     :maxdepth: 2
+    :hidden:
 
     getting_started/index
     user_guide/index