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/plugins/notifications/slack.md

@@ -78,7 +78,7 @@ npm install medusa-plugin-slack-notification
 
 After that, open `medusa-config.js` and add the new plugin with its configurations in the `plugins` array:
 
-```jsx title=medusa-config.js
+```js title=medusa-config.js
 const plugins = [
   // ...,
   {