Update color and background-color utilities

- Adds new functions to generate additional Sass maps
- Adds new root variables for rgb() versions of our theme colors, plus a few extras
- Adds ability to change the alpha transparency of text color and background color utilities with new utilities, inline styles, or local CSS var
- Updates documentation for color and background-color utilities pages
- Deprecates .text-black-50 and .text-white-50 since those (and additional transparency levels) can be generated on the fly

Change approach slightly to prevent cascade

scss/_root.scss

@@ -8,6 +8,14 @@
     --#{$variable-prefix}#{$color}: #{$value};
   }
 
+  @each $color, $value in $theme-colors-rgb {
+    --#{$variable-prefix}#{$color}-rgb: #{$value};
+  }
+
+  --#{$variable-prefix}white-rgb: #{to-rgb($white)};
+  --#{$variable-prefix}black-rgb: #{to-rgb($black)};
+  --#{$variable-prefix}body-rgb: #{to-rgb($body-color)};
+
   // Use `inspect` for lists so that quoted items keep the quotes.
   // See https://github.com/sass/sass/issues/2383#issuecomment-336349172
   --#{$variable-prefix}font-sans-serif: #{inspect($font-family-sans-serif)};