[SPARK-24507][DOCUMENTATION] Update streaming guide

## What changes were proposed in this pull request?
Updated streaming guide for direct stream and link to integration guide.

## How was this patch tested?
jekyll build

Author: Rekha Joshi <rekhajoshm@gmail.com>

Closes #21683 from rekhajoshm/SPARK-24507.

(cherry picked from commit f599cde)
Signed-off-by: hyukjinkwon <gurwls223@apache.org>

docs/streaming-programming-guide.md

@@ -2176,6 +2176,8 @@ the input data stream (using `inputStream.repartition(<number of partitions>)`).
 This distributes the received batches of data across the specified number of machines in the cluster
 before further processing.
 
+For direct stream, please refer to [Spark Streaming + Kafka Integration Guide](streaming-kafka-integration.html)
+
 ### Level of Parallelism in Data Processing
 {:.no_toc}
 Cluster resources can be under-utilized if the number of parallel tasks used in any stage of the