Consider dropping prepend(), in favor of using join() directly:

In doing so we can eliminate https://github.com/google/docsy/blob/main/assets/scss/support/_functions.scss.

AFAICT, there is only one instance of the call to prepend() in Docsy SCSS.

The other function that was in this file (color-diff()) was removed by:

• [BSv5] replace lighten() by tint-color(), and var/func clean up #1384