docs: update endpoints to use file-routing approach (#5397)

- Move the original guides for creating endpoints and middlewares to sub-sections in the Endpoints category.
- Replace existing guides for endpoints and middlewares with the new approach.
- Update all endpoints-related snippets across docs to use this new approach.

www/apps/docs/content/development/api-routes/example-logged-in-user.mdx

@@ -0,0 +1,171 @@
+---
+addHowToData: true
+---
+
+import Troubleshooting from '@site/src/components/Troubleshooting'
+import ServiceLifetimeSection from '../../troubleshooting/awilix-resolution-error/_service-lifetime.md'
+import CustomRegistrationSection from '../../troubleshooting/awilix-resolution-error/_custom-registration.md'
+
+# Example: Access Logged-In User
+
+This document gives an example of how you can use middlewares to register the logged-in user in the dependency container of your commerce application. You can then access the logged-in user in other resources, such as services.
+
+:::tip
+
+You can apply the same steps if you want to register the current customer.
+
+:::
+
+Learn more about [middlewares in its guide](./add-middleware.mdx).
+
+## Step 1: Create the Middleware
+
+Create the file `src/api/middlewares.ts` with the following content:
+
+```ts title=src/api/middlewares.ts
+import type { 
+  MiddlewaresConfig, 
+  User, 
+  UserService,
+} from "@medusajs/medusa"
+import type { 
+  MedusaNextFunction, 
+  MedusaRequest, 
+  MedusaResponse,
+} from "@medusajs/medusa"
+
+const registerLoggedInUser = async (
+  req: MedusaRequest, 
+  res: MedusaResponse, 
+  next: MedusaNextFunction
+) => {
+  let loggedInUser: User | null = null
+
+  if (req.user && req.user.userId) {
+    const userService = 
+      req.scope.resolve("userService") as UserService
+    loggedInUser = await userService.retrieve(req.user.userId)
+  }
+
+  req.scope.register({
+    loggedInUser: {
+      resolve: () => loggedInUser,
+     },
+   })
+  
+  next()
+}
+
+export const config: MiddlewaresConfig = {
+  routes: [
+    {
+      matcher: "/admin/products",
+      middlewares: [registerLoggedInUser],
+    },
+  ],
+}
+```
+
+This creates a `registerLoggedInUser` middleware that handles registering the logged-in user in the dependency container.
+
+It then applies this middleware on the `/admin/products` API Route. You can change that to be the route of any other API Route or a pattern that matches multiple routes.
+
+---
+
+## Step 2: Use in a Service
+
+Access the logged-in user resource in the constructor of a service:
+
+<!-- eslint-disable prefer-rest-params -->
+
+```ts
+import { Lifetime } from "awilix"
+import { 
+  TransactionBaseService, 
+  User,
+} from "@medusajs/medusa"
+
+class HelloService extends TransactionBaseService {
+
+  protected readonly loggedInUser_: User | null
+
+  constructor(container, options) {
+    super(...arguments)
+
+    try {
+      this.loggedInUser_ = container.loggedInUser
+    } catch (e) {
+      // avoid errors when backend first runs
+    }
+  }
+
+  // ...
+}
+
+export default HelloService
+```
+
+If you're accessing it in an extended core service, it’s important to change the lifetime of the service to `Lifetime.SCOPED`. For example:
+
+<!-- eslint-disable prefer-rest-params -->
+
+```ts
+import { Lifetime } from "awilix"
+import { 
+  ProductService as MedusaProductService, 
+  User, 
+} from "@medusajs/medusa"
+
+// extend core product service
+class ProductService extends MedusaProductService {
+  // The default life time for a core service is SINGLETON
+  static LIFE_TIME = Lifetime.SCOPED
+
+  protected readonly loggedInUser_: User | null
+
+  constructor(container, options) {
+    super(...arguments)
+
+    this.loggedInUser_ = container.loggedInUser
+  }
+}
+
+export default ProductService
+```
+
+You can learn more about the importance of changing the service lifetime in the [Middlewares documentation](./add-middleware.mdx#note-about-services-lifetime).
+
+---
+
+## Step 3: Test it Out
+
+To test out your implementation, run the following command in the root directory of the Medusa backend to transpile your changes:
+
+```bash npm2yarn
+npm run build
+```
+
+Then, run your backend with the following command:
+
+```bash npm2yarn
+npx medusa develop
+```
+
+If you try accessing the API Routes you added the middleware to, you should see your implementation working as expected.
+
+---
+
+## Troubleshooting
+
+<Troubleshooting 
+  sections={[
+    {
+      title: 'AwilixResolutionError: Could Not Resolve X',
+      content: <ServiceLifetimeSection />
+    },
+    {
+      title: 'AwilixResolutionError: Could Not Resolve X (Custom Registration)',
+      content: <CustomRegistrationSection />
+    }
+  ]}
+/>