docs: revise main docs outline (#10502)

www/apps/book/app/learn/fundamentals/api-routes/page.mdx

@@ -0,0 +1,65 @@
+export const metadata = {
+  title: `${pageNumber} API Routes`,
+}
+
+# {metadata.title}
+
+In this chapter, you’ll learn what API Routes are and how to create them.
+
+## What is an API Route?
+
+An API Route is an endpoint. It exposes commerce features to external applications, such as storefronts, the admin dashboard, or third-party systems.
+
+The Medusa core application provides a set of admin and store API routes out-of-the-box. You can also create custom API routes to expose your custom functionalities.
+
+---
+
+## How to Create an API Route?
+
+An API Route is created in a TypeScript or JavaScript file under the `src/api` directory of your Medusa application. The file’s name must be `route.ts` or `route.js`. 
+
+![Example of API route in the application's directory structure](https://res.cloudinary.com/dza7lstvk/image/upload/v1732808645/Medusa%20Book/route-dir-overview_dqgzmk.jpg)
+
+Each file exports API Route handler functions for at least one HTTP method (`GET`, `POST`, `DELETE`, etc…).
+
+For example, to create a `GET` API Route at `/hello-world`, create the file `src/api/hello-world/route.ts` with the following content:
+
+```ts title="src/api/hello-world/route.ts"
+import type {
+  MedusaRequest,
+  MedusaResponse,
+} from "@medusajs/framework/http"
+
+export const GET = (
+  req: MedusaRequest,
+  res: MedusaResponse
+) => {
+  res.json({
+    message: "[GET] Hello world!",
+  })
+}
+```
+
+### Test API Route
+
+To test the API route above, start the Medusa application:
+
+```bash npm2yarn
+npm run dev
+```
+
+Then, send a `GET` request to the `/hello-world` API Route:
+
+```bash
+curl http://localhost:9000/hello-world
+```
+
+---
+
+## When to Use API Routes
+
+<Note title="Use API routes when" type="success">
+
+You're exposing custom functionality to be used by a storefront, admin dashboard, or any external application.
+
+</Note>