minor #13861 add a note about npm commands equivalent to yarn's (nabbisen)

This PR was submitted for the 5.1 branch but it was merged into the 4.4 branch instead.

Discussion
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add a note about npm commands equivalent to yarn's

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I considered about which style was good to read.
Finally, I used code-in-note style, which was used in [Sub-Domain Routing](https://symfony.com/doc/current/routing.html#sub-domain-routing) section.

Commits
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d084e99 add a note about npm commands equivalent to yarn's

Author: javiereguiluz

frontend/encore/simple-example.rst

@@ -74,6 +74,21 @@ To build the assets, run:
 
     Stop and restart ``encore`` each time you update your ``webpack.config.js`` file.
 
+.. note::
+
+    If you prefer npm, run these commands instead:
+    
+    .. code-block:: terminal
+
+        # compile assets once
+        $ npm run dev
+
+        # or, recompile assets automatically when files change
+        $ npm run watch
+
+        # on deploy, create a production build
+        $ npm run build
+
 Congrats! You now have three new files:
 
 * ``public/build/app.js`` (holds all the JavaScript for your "app" entry)