[jvm-packages] Scaladoc issues with 2.12

[thesuperzapper]

As we are moving to Scala 2.12 exclusivly, I can see we have moved to the new Scaladoc, however there seems to be something wrong, as the new layout is significantly harder to use.

XGBoost4j Docs: https://xgboost.readthedocs.io/en/latest/jvm/scaladocs/xgboost4j/index.html
XGBoost4j-spark Docs: https://xgboost.readthedocs.io/en/latest/jvm/scaladocs/xgboost4j-spark/index.html

Both of which are not very usefull because they hide all classes behind 5+ clicks, for example:

Perhaps we could link deeper into the classpath (from the xgboost.ai site), or change something else about how we are using Scaladoc with 2.12?

Deeper link example: https://xgboost.readthedocs.io/en/latest/jvm/scaladocs/xgboost4j-spark/ml/dmlc/xgboost4j/scala/spark/index.html

[hcho3]

Is it possible to use older theme for the doc? I liked the old theme a lot better.

@CodingCat @trams

[trams]

A agree that having to click 5 times to get to any of the classes is not good.
I would gladly help but I know nothing about how we generate scaladoc in xgboost. If someone could get pointers I may help

[hcho3]

@trams Here is the script we run to generate scaladoc:

xgboost/tests/ci_build/build_jvm_doc.sh

Lines 19 to 24 in b9b57f2

	# Install JVM packages in local Maven repository
	mvn --no-transfer-progress install -DskipTests
	# Build Scaladocs
	mvn --no-transfer-progress scala:doc -DskipTests
	# Build Javadocs
	mvn --no-transfer-progress javadoc:javadoc -DskipTests

[CodingCat]

comparing to scala's own doc https://www.scala-lang.org/api/current/

I think a quick fix is to provide a rootdoc.txt https://github.com/scala/scala/blob/1e904d75450373d7845eb7452d940e52d955b27d/src/library/rootdoc.txt

https://stackoverflow.com/questions/53566250/how-do-i-configure-maven-to-use-rootdoc-txt-in-scaladoc-report