Updates to guides after RailsGirls Canberra (and some minor functionality changes!)

I just participated as a Coach for RailsGirls Canberra and had a few notes for the guides which I thought might benefit from some updates.

These should hopefully be pretty straight forward (mainly content, some tweaks and documenting around some snags we found over the course of the day), but I did also add some javascript to automatically detect the OS and switch the commands - this step was surprisingly often overlooked by the participants.

Thanks for setting up such a great foundation of content!