document use of dark mode images for images with meaningful color

[ErinBecker]

Automatic conversion of images to dark mode was added to {varnish} in carpentries/varnish#153. There are specific instances in which it doesn't make sense to display a dark mode version of the image, as for example when the image is a screencapture of a tool output where the color scheme is meaningful. This is demonstrated in the images below, which show a FastQC screenshot in the Data Carpentry Genomics curriculum (above) and the corresponding dark-i-fied image (below).

After discussion with @tobyhodges and @froggleston, we think the right approach to take for situations like this is to prompt Maintainers to explicitly set the light mode version of the image as the dark mode image. Leaving this note here to make sure we add this to documentation and communicate with Maintainers before the next {varnish} release.