Document how to upgrade React tokens

[srambach]

Is this a new page or a change to an existing one?
We need to give some guidance on how to handle upgrading React tokens. They should be pointing to the var rather than the value so that dark theme will work properly.

For things like breakpoints, spacers, and widths we may be able to make a reliable substitution with a code mod.

For colors, let's add a code mod pointing to a new "placeholder" token set to a hot pink highlight color. This will make it easy to find places where semantic color tokens need to be selected.

Here's a start to the information that could be on this page. https://docs.google.com/document/d/1kjqv-gyuf2k9fewlUpnV3kpCmFbM_q6YhutPsjlOu88/edit?usp=sharing