wip: doc for color scales

[mattijn]

fix: https://github.com/altair-viz/altair/issues/2861 preview: https://mattijn.github.io/altair-docs/user_guide/color_usage.html

[mattijn]

I'm struggling a bit with this one. @joelostblom, can you have a look if this is going in the right direction? Its maybe a bit too lengthy. I also doubt on the order of the sections. Currently the page starts with some color-theory including examples of good vs bad practices to continue with the predefined color schemes in Altair. I can imagine that one does not want to scroll through the theory each time to reach the available color schemes in Altair..

[joelostblom]

Sorry for the delay in reviewing this @mattijn I had a go now but something is up with my environment, so I will try again tomorrow or during next week.