This adds some design tokens for standard side panels. A side panel is a

component that will be placed on the side of a container, and can be collapsed

and expanded. This is useful for things like navigation or tool menus.
In Review Board, we'll be using these definitions in the PDF navigation side

panel in the PDF review UI. The screenshots show these off, with an active

first item in the sidebar and hovered over the third item.

Tested using these tokens for the PDF review UI.
Made sure that all bg colours (normal, active and hover) have WCAG AA

compliant contrast with fg colours.