Next steps for notebooks and tutorials

With #59 we have good infrastructure for generating notebooks. There are a couple more steps we need:

* Content. This could be taken from existing open-source tutorials (#68) and we should also port existing notebooks like [these](https://github.com/mikeinnes/flux-notebooks) and [these](https://github.com/aviks/mltraining).
* Auto build notebooks – we can do this on our CI, like documenter, and push them to a branch of this repo.
* JuliaBox – really just the same as the above, but push them to the [JuliaBoxTutorials](https://github.com/JuliaComputing/JuliaBoxTutorials) repo, which gets synced to JuliaBox users' drives.
* Flux Website – we should have a tutorials section on the website, built from these notebooks. We'll need to build HTML from the notebooks we have (ideally with niceties like ToC, but that's secondary) and push them to the Flux website's github pages.

Useful resources:

* [PyTorch tutorials](https://pytorch.org/tutorials/)
* [MXNet – The Straight Dope](https://gluon.mxnet.io)
* Knet [tutorials](https://github.com/denizyuret/Knet.jl/tree/master/tutorial) and [examples](https://github.com/denizyuret/Knet.jl/tree/master/examples)

Start by reviewing issues #59 and #68, then inspect the linked Flux, JuliaBox, PyTorch, MXNet, and Knet tutorial resources. Define the scope before choosing an entry point: the issue describes notebook content, CI builds, JuliaBox synchronization, and Flux website publishing. Done would require an agreed plan or implementation covering the selected steps.

Written by the indexing model from the issue text.