[SPARK-16046][DOCS] Aggregations in the Spark SQL programming guide

## What changes were proposed in this pull request?

- A separate subsection for Aggregations under “Getting Started” in the Spark SQL programming guide. It mentions which aggregate functions are predefined and how users can create their own.
- Examples of using the `UserDefinedAggregateFunction` abstract class for untyped aggregations in Java and Scala.
- Examples of using the `Aggregator` abstract class for type-safe aggregations in Java and Scala.
- Python is not covered.
- The PR might not resolve the ticket since I do not know what exactly was planned by the author.

In total, there are four new standalone examples that can be executed via `spark-submit` or `run-example`. The updated Spark SQL programming guide references to these examples and does not contain hard-coded snippets.

## How was this patch tested?

The patch was tested locally by building the docs. The examples were run as well.

![image](https://cloud.githubusercontent.com/assets/6235869/21292915/04d9d084-c515-11e6-811a-999d598dffba.png)

Author: aokolnychyi <okolnychyyanton@gmail.com>

Closes #16329 from aokolnychyi/SPARK-16046.