minor #9351 Adding a documentation page about Bootstrap 4 form theme (Nyholm)

This PR was squashed before being merged into the master branch (closes #9351).

Discussion
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Adding a documentation page about Bootstrap 4 form theme

The Boostrap 4 theme has some "special" feature that we should mention.

* I wanted to give the user a copy-paste get started template.
* I mention WCAG2.0 because we should be proud of that and make users aware of it
* The special classes in "custom forms" are not boostrap classes. They are feature flags in the Symfony boostrap 4 theme.
* The half "labels and erros"  is copied from the `form_custimazation.rst`.

Commits
-------

696a366 Adding a documentation page about Bootstrap 4 form theme

Author: javiereguiluz

form/bootstrap4.rst

@@ -0,0 +1,116 @@
+Bootstrap 4 Form Theme
+======================
+
+Symfony provides several ways of integrating Bootstrap into your application. The
+most straightforward way is to just add the required ``<link>`` and ``<script>``
+elements in your templates (usually you only include them in the main layout
+template which other templates extend from):
+
+.. code-block:: html+twig
+
+    {# templates/base.html.twig #}
+
+    {# beware that the blocks in your template may be named different #}
+    {% block head_css %}
+        <!-- Copy CSS from https://getbootstrap.com/docs/4.0/getting-started/introduction/#css -->
+    {% endblock %}
+    {% block head_js %}
+        <!-- Copy JavaScript from https://getbootstrap.com/docs/4.0/getting-started/introduction/#js -->
+    {% endblock head %}
+
+If your application uses modern front-end practices, it's better to use
+:doc:`Webpack Encore </frontend>` and follow :doc:`this tutorial </frontend/encore/bootstrap>`
+to import Bootstrap's sources into your SCSS and JavaScript files.
+
+The next step is to configure the Symfony application to use Bootstrap 4 styles
+when rendering forms. If you want to apply them to all forms, define this
+configuration:
+
+.. configuration-block::
+
+    .. code-block:: yaml
+
+        # config/packages/twig.yaml
+        twig:
+            form_themes: ['bootstrap_4_layout.html.twig']
+
+    .. code-block:: xml
+
+        <!-- config/packages/twig.xml -->
+        <?xml version="1.0" encoding="UTF-8" ?>
+        <container xmlns="http://symfony.com/schema/dic/services"
+            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+            xmlns:twig="http://symfony.com/schema/dic/twig"
+            xsi:schemaLocation="http://symfony.com/schema/dic/services
+                http://symfony.com/schema/dic/services/services-1.0.xsd
+                http://symfony.com/schema/dic/twig
+                http://symfony.com/schema/dic/twig/twig-1.0.xsd">
+
+            <twig:config>
+                <twig:form-theme>bootstrap_4_layout.html.twig</twig:form-theme>
+                <!-- ... -->
+            </twig:config>
+        </container>
+
+    .. code-block:: php
+
+        // config/packages/twig.php
+        $container->loadFromExtension('twig', array(
+            'form_themes' => array(
+                'bootstrap_4_layout.html.twig',
+            ),
+
+            // ...
+        ));
+
+If you prefer to apply the Bootstrap styles on a form to form basis, include the
+``form_theme`` tag in the templates where those forms are used:
+
+.. code-block:: twig
+
+    {# ... #}
+    {# this tag only applies to the forms defined in this template #}
+    {% form_theme form 'bootstrap_4_layout.html.twig' %}
+
+    {% block body %}
+        <h1>User Sign Up:</h1>
+        {{ form(form) }}
+    {% endblock body %}
+
+Accessibility
+-------------
+
+The Bootstrap 4 framework has done a good job making it accessible for functional
+variations like impaired vision and cognitive ability. Symfony has taken this one
+step further to make sure the form theme complies with the `WCAG 2.0 standard`_.
+
+This does not mean that your entire website automatically complies with the full
+standard, but it does mean that you have come far in your work to create a design
+for **all** users.
+
+Custom Forms
+------------
+
+Bootstrap 4 has a feature called "`custom forms`_". You can enable that on your
+Symfony Form ``RadioType`` and ``CheckboxType`` by adding a class called ``radio-custom``
+and ``checkbox-custom`` respectively.
+
+.. code-block:: html+twig
+
+    {{ form_row(form.myRadio, {attr: {class: 'radio-custom'} }) }}
+    {{ form_row(form.myCheckbox, {attr: {class: 'checkbox-custom'} }) }}
+
+Labels and Errors
+-----------------
+
+When you use the Bootstrap form themes and render the fields manually, calling
+``form_label()`` for a checkbox/radio field doesn't render anything. Due to Bootstrap
+internals, the label is already rendered by ``form_widget()``.
+
+Form errors are rendered **inside** the ``<label>`` element to make sure there
+is a strong connection between the error and its ``<input>``, as required by the
+`WCAG 2.0 standard`_.
+
+.. _`their documentation`: https://getbootstrap.com/docs/4.0/
+.. _`WCAG 2.0 standard`: https://www.w3.org/TR/WCAG20/
+.. _`custom forms`: https://getbootstrap.com/docs/4.0/components/forms/#custom-forms

form/form_customization.rst

@@ -127,6 +127,10 @@ fragment needed to render every part of a form:
     calling ``form_label()`` for a checkbox/radio field doesn't show anything.
     Due to Bootstrap internals, the label is already shown by ``form_widget()``.
 
+.. tip::
+
+    Read more about the :doc:`Bootstrap 4 form theme </form/bootstrap4>`.
+
 In the next section you will learn how to customize a theme by overriding
 some or all of its fragments.