minor #9980 Popper js is required (Etilawin)

This PR was submitted for the 4.1 branch but it was squashed and merged into the 3.4 branch instead (closes #9980).

Discussion
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Popper js is required

If you don't add popper js you will probably have the following message while compiling with yarn

This dependency was not found:
* popper.js in ./node_modules/bootstrap/dist/js/bootstrap.js

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Commits
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a09797a Popper js is required

Author: javiereguiluz

frontend/encore/bootstrap.rst

@@ -37,11 +37,12 @@ file into ``global.scss``. You can even customize the Bootstrap variables first!
 Importing Bootstrap JavaScript
 ------------------------------
 
-Bootstrap JavaScript requires jQuery, so make sure you have this installed:
+Bootstrap JavaScript requires jQuery and Popper.js, so make sure you have this installed:
 
 .. code-block:: terminal
 
     $ yarn add jquery --dev
+    $ yarn add popper.js --dev
 
 Next, make sure to call ``.autoProvidejQuery()`` in your ``webpack.config.js`` file: