minor #9614 Add @a2lix/symfony-collection solution (webda2l)

This PR was squashed before being merged into the 3.4 branch (closes #9614).

Discussion
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Add @a2lix/symfony-collection solution

Hello,

I wrote a blog post earlier this month about this package: https://a2lix.fr/2018/04/05/manage-symfony-form-collection-with-vanilla-javascript.html.
IMO, as there are Symfony best practices, we should do a bit of Javascript good practices and start discouraging the use of jQuery in the documentation right now.

I thought about updating the different parts of jQuery code of this form_collections.rst to a modern JS code, but it doesn't satisfy me because to do good things, it will increase number of lines of JS.
And, IMO, it's better to push people to use an external JS solution (or create symfony JS based solution) instead of explain quickly how redo a poor JS solution.

Any thoughts?

Commits
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d2b7cee Add @a2lix/symfony-collection solution

Author: javiereguiluz

form/form_collections.rst

@@ -714,13 +714,17 @@ the relationship between the removed ``Tag`` and ``Task`` object.
     updated (whether you're adding new tags or removing existing tags) on
     each Tag object itself.
 
-.. sidebar:: Form collection jQuery plugin
+.. sidebar:: Form collection external solutions
 
-    The jQuery plugin  `symfony-collection`_ helps with ``collection`` form elements,
-    by providing the JavaScript functionality needed to add, edit and delete
-    elements of the collection. More advanced functionality like moving or duplicating
-    an element in the collection and customizing the buttons is also possible.
+    The package `@a2lix/symfony-collection`_ helps with ``collection`` form elements,
+    by providing to modern browsers the Javascript functionality needed to add, edit and delete
+    elements of the collection.
+
+    If you want to use jQuery, there is a jQuery extension, `symfony-collection`_ with
+    more advanced functionality like moving or duplicating an element in the collection
+    and customizing the buttons is also possible.
 
 .. _`Owning Side and Inverse Side`: https://www.doctrine-project.org/projects/doctrine-orm/en/current/reference/unitofwork-associations.html
 .. _`JSFiddle`: http://jsfiddle.net/847Kf/4/
+.. _`@a2lix/symfony-collection`: https://github.com/a2lix/symfony-collection
 .. _`symfony-collection`: https://github.com/ninsuo/symfony-collection