[SPARK-10061] [DOC] ML ensemble docs

User guide for spark.ml GBTs and Random Forests.
The examples are copied from the decision tree guide and modified to run.

I caught some issues I had somehow missed in the tree guide as well.

I have run all examples, including Java ones.  (Of course, I thought I had previously as well...)

CC: mengxr manishamde yanboliang

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #8369 from jkbradley/ml-ensemble-docs.