[docs] Add reasoning for no #000000 backgrounds in theming

[hellogreg]

Since it's come up a few times, maybe we should write a sentence or two on our site about why we don't offer a pure black background in our darkest theme.

Even though white text on a black background passes WCAG conformance (there's no maximum contrast threshold), that doesn't make it accessible--as it can be tough to read.

From Benny:

Full black has limited use in RHDS, but for use in backgrounds, ...

From Mark:

I can see this info on Foundations > Color. And, cross link from the Tokens page where needed.