Improve Windows entry in generated release page

[carver]

The file should be a .zip instead of a .tar.gz. Also, we need a new Windows icon. Though it was removed for licensing/trademark reasons, as I understand it, so finding a suitable replacement that matches the style may be a challenge.

I did some manual fixups for these in the v0.1.0 release, but it would be nice to get these set up to work automatically for future releases.