`@theme-ui/color-modes` lacks documentation

[hasparus]

I feel that we should advise people to combine low-level Theme UI packages to use only what they need. For example, most folks won't need MDX Provider. Some Theme UI users don't even use color modes. Some people won't use Components and will instead React-Aria, Radix UI or Reakit.

@theme-ui/color-modes and @theme-ui/theme-provider could use defining what's public API in them and what's not. We could then include the public surface in the docs.