[MINOR][SPARKR] Move 'Data type mapping between R and Spark' to right place in SparkR doc.

Section ```Data type mapping between R and Spark``` was put in the wrong place in SparkR doc currently, we should move it to a separate section.

## What changes were proposed in this pull request?
Before this PR:
![image](https://cloud.githubusercontent.com/assets/1962026/24340911/bc01a532-126a-11e7-9a08-0d60d13a547c.png)

After this PR:
![image](https://cloud.githubusercontent.com/assets/1962026/24340938/d9d32a9a-126a-11e7-8891-d2f5b46e0c71.png)

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #17440 from yanboliang/sparkr-doc.