Review/update ND4J user guide

[AlexDBlack]

https://nd4j.org/userguide

I don't think this has been updated in quite a while.
One thing I noticed is that the index accumulation ops section doesn't use the convenience methods INDArray.argmax etc.
We've also got a ton of new ops that we should probably mention here.

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[crockpotveggies]

Going to close this because our ND4J website will be forwarded to DL4J. I'll set that up right now.

[AlexDBlack]

Forwarding is a good idea 👍

However, I think this should stay open: https://deeplearning4j.org/docs/latest/nd4j-overview
It's the same content, and the same issues apply - i.e., it hasn't been thoroughly reviewed/updated in a while.