docs: improve commerce modules [1/n] (#9498)

Improve and add docs for API Key, Auth, and Cart Modules

[1/n]

www/apps/resources/sidebar.mjs

@@ -39,8 +39,8 @@ export const sidebar = sidebarAttachHrefCommonOptions([
               },
               {
                 type: "link",
-                path: "/commerce-modules/api-key/relations-to-other-modules",
-                title: "Relation to Modules",
+                path: "/commerce-modules/api-key/links-to-other-modules",
+                title: "Link to Modules",
               },
             ],
           },
@@ -223,6 +223,11 @@ export const sidebar = sidebarAttachHrefCommonOptions([
             path: "/commerce-modules/cart/examples",
             title: "Examples",
           },
+          {
+            type: "link",
+            path: "/commerce-modules/cart/extend",
+            title: "Extend Module",
+          },
           {
             type: "sub-category",
             title: "Concepts",
@@ -244,8 +249,8 @@ export const sidebar = sidebarAttachHrefCommonOptions([
               },
               {
                 type: "link",
-                path: "/commerce-modules/cart/relations-to-other-modules",
-                title: "Relations to Other Modules",
+                path: "/commerce-modules/cart/links-to-other-modules",
+                title: "Links to Other Modules",
               },
             ],
           },