[BLOCKING] Adding JVM doc build to Jenkins CI (#3567)

* Adding Java/Scala doc build to Jenkins CI

* Deploy built doc to S3 bucket

* Build doc only for branches

* Build doc first, to get doc faster for branch updates

* Have ReadTheDocs download doc tarball from S3

* Update JVM doc links

* Put doc build commands in a script

* Specify Spark 2.3+ requirement for XGBoost4J-Spark

* Build GPU wheel without NCCL, to reduce binary size

jvm-packages/build_doc.sh

@@ -0,0 +1,34 @@
+#!/bin/bash
+
+if [ $# -ne 1 ]; then
+  echo "Usage: $0 [commit id]"
+  exit 1
+fi
+
+set -e
+set -x
+
+commit_id=$1
+
+# Install JVM packages in local Maven repository
+mvn install -DskipTests
+# Build Scaladocs
+mvn scala:doc -DskipTests
+# Build Javadocs
+mvn javadoc:javadoc -DskipTests
+
+# Package JVM docs in a tarball
+mkdir -p tmp/scaladocs
+cp -rv xgboost4j/target/site/apidocs/ ./tmp/javadocs/
+cp -rv xgboost4j/target/site/scaladocs/ ./tmp/scaladocs/xgboost4j/
+cp -rv xgboost4j-spark/target/site/scaladocs/ ./tmp/scaladocs/xgboost4j-spark/
+cp -rv xgboost4j-flink/target/site/scaladocs/ ./tmp/scaladocs/xgboost4j-flink/
+
+cd tmp
+tar cvjf ${commit_id}.tar.bz2 javadocs/ scaladocs/
+mv ${commit_id}.tar.bz2 ..
+cd ..
+rm -rfv tmp/
+
+set +x
+set +e