docs: document tax-inclusive promotions concept (#12849)

www/apps/resources/sidebars/promotion.mjs

@@ -22,6 +22,11 @@ export const promotionSidebar = [
             path: "/commerce-modules/promotion/concepts",
             title: "Promotion",
           },
+          {
+            type: "link",
+            path: "/commerce-modules/promotion/promotion-taxes",
+            title: "Tax-Inclusive Promotions",
+          },
           {
             type: "link",
             path: "/commerce-modules/promotion/application-method",