Reduce the size of the docs site files

[jessestorbeck]

I noticed this warning when deploying an update to the docs:

Looking at the docs files in the gh-pages branch, most of the artifact size is coming from the assets for version 3.6, which had a lot of duplicates of large gifs (these came from the previous Gitbook docs). This bloat was significantly decreased for v4.0, but 3.6's files remain unchanged in gh-pages (this is just how mike works; previous versions are unaffected when you deploy a new version).

When we eventually try to deploy docs v4.1, I worry the deployment will fail, since I'm guessing we'll be about 250 MB over the 1 GB limit. We may need to either (a) remove v3.6 or (b) manually remove some of the duplicate gifs that aren't being used in v3.6. I'd support the former, since it will be laborious to look through which image versions are actually used in the old docs (e.g. in some cases, if you see ubuntu7.gif and ubuntu7 (1).gif, it's actually the latter that's being referenced). We could also just wait and see if the next deploy fails before doing anything.

This is probably also an argument against overusing the screencap gifs in the future, since they are fairly large. We might also want to compress our site images for future docs versions.