Update code examples on page: "Building Neural Networks with DeepLearning4J"

[AlexDBlack]

Code examples here contain the old string-based API for setting activation functions that has been removed some releases ago.
.activation("tanh")

https://deeplearning4j.org/building-neural-net-with-dl4j

Edit: it would also be good to add some information on how the global settings + layer overrides work.

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