[Fedora-join] Easy fix: add F32 wallpaper to the wiki

[Alessio]

Hello. Here we have an easy fix task: add the F32 wallpaper to the
wiki.
It is nice to keep track of all the awesome wallpapers used in every
release.

Please refer to this ticket:
https://pagure.io/fedora-marketing/issue/300


Thanks,
A.