docs: added a services constraint document (#8999)

- Added a services constraint document mentioning that a service's methods must be async.
- Modify the code in the services intro document to use `async` method.

www/apps/book/sidebar.mjs

@@ -150,6 +150,11 @@ export const sidebar = numberSidebarItems(
               path: "/advanced-development/modules/service-factory",
               title: "Service Factory",
             },
+            {
+              type: "link",
+              path: "/advanced-development/modules/service-constraints",
+              title: "Service Constraints",
+            },
             {
               type: "link",
               path: "/advanced-development/modules/isolation",