[jvm-packages] Tutorial of XGBoost4J-Spark (#3534)

* add back train method but mark as deprecated

* add back train method but mark as deprecated

* fix scalastyle error

* fix scalastyle error

* add new

* update doc

* finish Gang Scheduling

* more

* intro

* Add sections: Prediction, Model persistence and ML pipeline.

* Add XGBoost4j-Spark MLlib pipeline example

* partial finished version

* finish the doc

* adjust code

* fix the doc

* use rst

* Convert XGBoost4J-Spark tutorial to reST

* Bring XGBoost4J up to date

* add note about using hdfs

* remove duplicate file

* fix descriptions

* update doc

* Wrap HDFS/S3 export support as a note

* update

* wrap indexing_mode example in code block

doc/jvm/index.rst

@@ -145,8 +145,10 @@ Contents
 ********
 
 .. toctree::
+  :maxdepth: 2
 
-  Java Overview Tutorial <java_intro>
+  java_intro
+  XGBoost4J-Spark Tutorial <xgboost4j_spark_tutorial>
   Code Examples <https://github.com/dmlc/xgboost/tree/master/jvm-packages/xgboost4j-example>
   XGBoost4J Java API <http://dmlc.ml/docs/javadocs/index.html>
   XGBoost4J Scala API <http://dmlc.ml/docs/scaladocs/xgboost4j/index.html>