Import color.rst
As discussed in swick/wayland-protocols#6 (closed), I am starting a new repository for hosting color and HDR related documentation to make it easier to find and contribute to it. If this repository gains enough traction, we may then consider moving it out of my personal group. I was assured that such move will not lose any history, issues, or MRs.
This MR imports the commits from https://gitlab.freedesktop.org/swick/wayland-protocols/-/commits/color/ and cleans up the directory structure leaving only color.rst and its pictures.