[SPARK-15863][SQL][DOC] Initial SQL programming guide update for Spark 2.0

## What changes were proposed in this pull request?

Initial SQL programming guide update for Spark 2.0. Contents like 1.6 to 2.0 migration guide are still incomplete.

We may also want to add more examples for Scala/Java Dataset typed transformations.

## How was this patch tested?

N/A

Author: Cheng Lian <lian@databricks.com>

Closes #13592 from liancheng/sql-programming-guide-2.0.