UX: Make category section link icons consistent with Sidebar's interface (#17922)

Instead of relying on another help to generate the icons, we want to
rely on the interface for adding prefix icons. This ensures that prefix
icons are consistent across the section links in Sidebar

lib/svg_sprite.rb

@@ -181,6 +181,7 @@ module SvgSprite
     "sign-out-alt",
     "signal",
     "sliders-h",
+    "square-full",
     "star",
     "step-backward",
     "step-forward",