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/file-service/local.md

@@ -80,7 +80,7 @@ If this configuration is not added, you’ll receive the error ["next/image Un-c
 
 In `next.config.js` add the following option in the exported object:
 
-```jsx title=next.config.js
+```js title=next.config.js
 const { withStoreConfig } = require("./store-config")
 
 // ...