adding link to spectrum styling docs (#3596)

Author: matthewdeutsch

packages/dev/docs/pages/react-spectrum/styling.mdx

@@ -61,7 +61,7 @@ All of the available style props are listed below.
 
 ## Dimension values
 
-Where applicable, each style property accepts Spectrum defined variables in addition to raw CSS values.
+Where applicable, each style property accepts [Spectrum defined variables](https://spectrum.adobe.com/page/design-tokens/#Size-tokens) in addition to raw CSS values.
 **Using Spectrum variables is preferred wherever possible**. These variables conform to Spectrum design
 defined sizing and spacing requirements, and also automatically adapt on different devices. For example,
 on touch screen devices, all Spectrum components expand in size to give users larger hit targets.