doc: GNOME example screenshot is old

[danth]

We should create a new screenshot for GNOME 45.

Also, the link crediting the original source of the wallpaper is now a 404.

[trueNAHO]

Alternatively, we could change the format to something like:

https://github.com/SenchoPens/base16.nix/blob/b390e87cd404e65ab4d786666351f1292e89162a/demo.gif