Consider adding generated content to .gitignore

[bsipocz]

So maintainers can add/update them with --force, but they won't show up as modified files for random contributors:

	modified:   content/mooreslaw_regression.csv
	modified:   content/mooreslaw_regression.npz
	modified:   content/tutorial-x-ray-image-processing/xray_image.gif

content/video/ also shows up as untracked files, probably should be added fully to .gitignore?

[rossbar]

In the longer term it might be nice to set up directories where inputs and generated outputs could live (e.g. data and _generated) and just ignore everything in those folders. However adding these to .gitignore would certainly be an improvement 👍 . In fact, it looks like the mooreslaw files shouldn't be tracked at all...

[bsipocz]

I've certainly seen arguments for keeping generated outputs around for notebooks that are supposed to run for a longer time, so wasn't sure which case these fall in to act on it.