Add credits for wallpapers

Currently, there are no direct credits to the artists of these wallpapers. This is generally frowned upon among artists, but it can make many other things difficult as well: - Tracking down the artists to find art of a similar style or support the artists - Finding and removing art if an artist asks for it - Determining which art is human-made and which is AI, and what AI model was used At the very least, linking to the uploads on Alphacoders, Wallhaven, etc. can make these easier. For a reference, I have credits in [my wallpaper repository](https://github.com/mimvoid/wallpaper-stash/blob/807a4f0f55a35be7cedb660a472db602680be4b5/wallpapers.md).