Additions to the color documentation

[DPX-designer]

I started by creating the "DevTools Exclusive Additions" section which lists the current DevTools specific color additions, Blue-55 etc.
To cater for DevTools (and even iOS) dark mode there is a good reason to extend the Photon colors to provide lighter alternatives. Perhaps we need to go to something like shade 20 for each color?
I made a start with:

• Blue-30
• Blue-20
• Magenta-40
• Red-40

I took note from the steps between Blue-50 to Blue-40 but it seems the step in saturation is different for each main color which is why I stopped for feedback. I wasn't able to gleam a "global pattern" to generate the remaining hues so we might need to consult the original photon color author for help with building up the lighter shades?

Let me know if there are additional DevTools specific colors I have missed!

@violasong @bwinton