Use the mechanism described here:
https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#specifying-the-theme-an-image-is-shown-to



to provide different logos for light and dark mode, so that the "Zephyr"
text is clearly visible in both.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>