Add 3D RGB is grayscale (and workarounds) to troubleshooting

[TimMonko]

I just tested the new 0.7.0a1 version, and saw that the Cat (RGB) example still switches to greyscale when switching to 3D.
By the name of the PR, I initially thought it would address this issue 😄
After reading the whole thread, thanks to @TimMonko & @Czaki, I understand it is probably a known bug related to napari/napari#3973! (Still posting this comment here in case people encounter the same issue as I did.)

Originally posted by @cnstt in napari/napari#8522 (comment)

RGB is rendered as grayscale in 3D, a limitation of Vispy. A workaround is to Split RGB in componenet layers and use blending modes like with non RGB images. This will display properly in 3D, but has limitations of... well, needing to represent data differently.