Rewrite chapter 3 with a ML systems focus

[profvjreddi]

The current chapter of the DL primer serves as an overview, but it was written rather hastily in preparation for class.

It needs to be completely reworked. Specifically, the chapter should focus on providing a solid background on neural networks, followed by a comprehensive perspective on the system. This approach will highlight the unique value we are adding, as there are already plentiful resources that explain vanilla neural networks in detail. But we want to build the bridge over to system issues.