minor #19792 [AssetMapper][Encore] document switching from AssetMapper to Encore (kbond)

This PR was merged into the 6.4 branch.

Discussion
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[AssetMapper][Encore] document switching from AssetMapper to Encore

There are some flex-related nuances when switching the default webapp from AssetMapper to Encore. This documents the proper steps to have a clean conversion.

Commits
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2c8bc90 [AssetMapper][Encore] document switching from AssetMapper to Encore

Author: javiereguiluz

frontend.rst

@@ -78,6 +78,26 @@ pre-processing CSS & JS and compiling and minifying assets.
 
 :doc:`Read the Encore Documentation </frontend/encore/index>`
 
+Switch from AssetMapper
+^^^^^^^^^^^^^^^^^^^^^^^
+
+By default, new Symfony webapp projects (created with ``symfony new --webapp myapp``)
+use AssetMapper. If you still need to use Webpack Encore, use the following steps to
+switch. This is best done on a new project and provides the same features (Turbo/Stimulus)
+as the default webapp.
+
+.. code-block:: terminal
+
+    # Remove AssetMapper & Turbo/Stimulus temporarily
+    $ composer remove symfony/ux-turbo symfony/asset-mapper symfony/stimulus-bundle
+
+    # Add Webpack Encore & Turbo/Stimulus back
+    $ composer require symfony/webpack-encore-bundle symfony/ux-turbo symfony/stimulus-bundle
+
+    # Install & Build Assets
+    $ npm install
+    $ npm run dev
+
 Stimulus & Symfony UX Components
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~