Update DL4J Spark training pages

[AlexDBlack]

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There is also some outdated content on there.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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[crockpotveggies]

@AlexDBlack I'm going to have a new place for you to put the updated doc on this branch here: https://github.com/deeplearning4j/deeplearning4j/tree/crockpot_overhaul

[AlexDBlack]

@crockpotveggies thanks. Let me know where.
I probably wont be able to get to actually writing/updating these docs until next week.