doc: refresh screenshots

[danth]

• All the wallpaper images are now properly licensed, with credit given to the original photographers
• Each screenshot showcases both a light and dark version
• The GNOME screenshot is updated from GNOME 44 to GNOME 46 - fixes #226

For future reference, the screenshots were edited as follows:

• Take screenshots from the testbed at its default resolution
• Load the images as layers in GIMP
• Select one half of the image (this is 640 pixels wide)
• Shear the selection by -150 in the X direction
• Delete the selected area from the top layer
• Delete the remaining small triangle in the bottom corner

Closes: #226