feature #13164 [Form] Mentioned the form theme for Foundation 6 (javiereguiluz)

This PR was squashed before being merged into the master branch.

Discussion
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[Form] Mentioned the form theme for Foundation 6

Fixes #12914
Fixes #13161

Commits
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261d67a [Form] Mentioned the form theme for Foundation 6

Author: OskarStark

form/form_themes.rst

@@ -35,7 +35,14 @@ in a single Twig template and they are enabled in the
   ``bootstrap_3_horizontal_layout.html.twig`` but updated for Bootstrap 4 styles.
 * `foundation_5_layout.html.twig`_, wraps each form field inside a ``<div>``
   element with the appropriate CSS classes to apply the default styles of the
-  `Foundation CSS framework`_.
+  version 5 of `Foundation CSS framework`_.
+* `foundation_6_layout.html.twig`_, wraps each form field inside a ``<div>``
+  element with the appropriate CSS classes to apply the default styles of the
+  version 6 of `Foundation CSS framework`_.
+
+.. versionadded:: 5.1
+
+    The ``foundation_6_layout.html.twig`` theme was introduced in Symfony 5.1.
 
 .. tip::
 
@@ -599,6 +606,7 @@ is a collection of fields (e.g. a whole form), and not just an individual field:
 .. _`Bootstrap 3 CSS framework`: https://getbootstrap.com/docs/3.3/
 .. _`Bootstrap 4 CSS framework`: https://getbootstrap.com/docs/4.1/
 .. _`foundation_5_layout.html.twig`: https://github.com/symfony/symfony/blob/master/src/Symfony/Bridge/Twig/Resources/views/Form/foundation_5_layout.html.twig
+.. _`foundation_6_layout.html.twig`: https://github.com/symfony/symfony/blob/master/src/Symfony/Bridge/Twig/Resources/views/Form/foundation_6_layout.html.twig
 .. _`Foundation CSS framework`: http://foundation.zurb.com/
 .. _`Twig "use" tag`: https://twig.symfony.com/doc/2.x/tags/use.html
 .. _`Twig parent() function`: https://twig.symfony.com/doc/2.x/functions/parent.html