[Documentation:Developer] Resize rainbowgrades.png (#574)

Fixes Submitty/Submitty/issues/10099

I am not too confident of whether image size is the reason, as I have
never had issue with it.
Yet I was able to compress the file, resizing from 4MB to 500KB.
Which should be faster to load.

Co-authored-by: Barb Cutler <[email protected]>

Author: ziesski