Update DL4J Spark training pages

Spark page needs some polish (I'm probably the best person to do this, or should at least be consulted on the technical aspects of this). IMO it should also be reorganized, and the parameter averaging implementation should be de-emphasized in the docs.
It may also be a good idea to create separate pages - something like:
(a) Intro and general issues (like memory, data, best practices, node failure/recovery, etc)
(b) Gradient sharing implementation
(c) Parameter averaging implementation

There is also some outdated content on there.

https://deeplearning4j.org/spark
https://deeplearning4j.org/distributed

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