Losslessly optimize PNG files

[lumiscosity]

Losslessly optimizes all of the PNG files in the repo. Done with:

oxipng -o max -a -s
oxipng -o max --zopfli -a -s

[rustbot]

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[Noratrieb]

oh that's pretty cool, I wasn't even aware that PNGs can be optimized by this much :3

[tshepang]

I did pause when I saw this pr, since it increases the size of the repo, but now I just thought of the great likelihood that the clones will be far less frequent than remote accesses.

[lumiscosity]

Yeah, my reasoning is that it'll save on fetch time on the rendered docs. Push comes to shove, you can always clone the repo with reduced history.