docs: added Override Tax Calculation Strategy doc (#3982)

www/docs/sidebars.js

@@ -939,6 +939,11 @@ module.exports = {
           id: "modules/taxes/backend/create-tax-provider",
           label: "Backend: Create Tax Provider",
         },
+        {
+          type: "doc",
+          id: "modules/taxes/backend/tax-calculation-strategy",
+          label: "Backend: Override Tax Calculation",
+        },
         {
           type: "link",
           href: "#",