[SPARK-16256][SQL][STREAMING] Added Structured Streaming Programming Guide

Title defines all.

Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #13945 from tdas/SPARK-16256.

docs/_layouts/global.html

@@ -73,6 +73,7 @@
                                 <li class="divider"></li>
                                 <li><a href="streaming-programming-guide.html">Spark Streaming</a></li>
                                 <li><a href="sql-programming-guide.html">DataFrames, Datasets and SQL</a></li>
+                                <li><a href="structured-streaming-programming-guide.html">Structured Streaming</a></li>
                                 <li><a href="mllib-guide.html">MLlib (Machine Learning)</a></li>
                                 <li><a href="graphx-programming-guide.html">GraphX (Graph Processing)</a></li>
                                 <li><a href="sparkr.html">SparkR (R on Spark)</a></li>