[Docs] Add caution alert when bundle require StimulusBundle

Author: ker0x

doc/index.rst

@@ -12,9 +12,11 @@ bringing the complexity of a React/Vue/Angular application.
 Installation
 ------------
 
-Before you start, make sure you have `StimulusBundle configured in your app`_.
+.. caution::
 
-Then install the bundle using Composer and Symfony Flex:
+    Before you start, make sure you have `StimulusBundle configured in your app`_.
+
+Install the bundle using Composer and Symfony Flex:
 
 .. code-block:: terminal