docs - add separate usage example for activations()

[obviousnic]

A minor issue in the docs for the activations() function in src/layers/basic.jl.

If a beginner following the previous examples in the docs, e.g.
loss(x, y) = crossentropy(m(x), y) + sum(norm, params(m))
and substituting activations() for params(), this will
result in the forward pass being computed twice, once when the model is evaluated, and again when activations is called.

The doc entry for activations() could provide an example or comment to emphasize that the model should not be called except through activations() itself.