[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

doc/conf.py

@@ -12,11 +12,18 @@
 # All configuration values have a default; values that are commented out
 # serve to show the default.
 from subprocess import call
+from sh.contrib import git
+import urllib.request
 from recommonmark.parser import CommonMarkParser
 import sys
 import os, subprocess
 import shlex
 import guzzle_sphinx_theme
+
+git_branch = str(git('rev-parse', '--abbrev-ref', 'HEAD')).rstrip('\n')
+filename, _ = urllib.request.urlretrieve('https://s3-us-west-2.amazonaws.com/xgboost-docs/{}.tar.bz2'.format(git_branch))
+call('if [ -d tmp ]; then rm -rf tmp; fi; mkdir -p tmp/jvm; cd tmp/jvm; tar xvf {}'.format(filename), shell=True)
+
 # If extensions (or modules to document with autodoc) are in another directory,
 # add these directories to sys.path here. If the directory is relative to the
 # documentation root, use os.path.abspath to make it absolute, like shown here.
@@ -94,6 +101,7 @@ autoclass_content = 'both'
 # List of patterns, relative to source directory, that match files and
 # directories to ignore when looking for source files.
 exclude_patterns = ['_build']
+html_extra_path = ['./tmp']
 
 # The reST default role (used for this markup: `text`) to use for all
 # documents.