minor #10926 Simplified the form theming docs (javiereguiluz)

This PR was squashed before being merged into the 4.1 branch (closes #10926).

Discussion
----------

Simplified the form theming docs

This is a total revamp of all the docs related to form themes and form customization. The current problems are:

* `form/form_customization.rst` and `form/rendering.rst` are too similar.
* `form/form_themes.rst` is incomplete and confusing
* Most of the form theming things are explained in `form/form_customization.rst`
  instead of `form/form_themes.rst`
* `reference/forms/twig_reference.rst` and `reference/twig_reference.rst`
  duplicate lots of contents

This PR does the following:

* Delete `form/rendering.rst` and merge it with `form/form_customization.rst`
* Remove anything related to form themes from `form/form_customization.rst`
* Add everything related to form themes to ``form/form_themes.rst``
* Remove the duplicated contents from `reference/twig_reference.rst` and link
  to the other form reference.

The result is:

* `form.rst`: main article explaining all the basics about forms.
* `form/form_customization.rst` explains everything about Form helper functions
  (`form_start()`, `form_row()`, etc.) and form variables (`required`, `compound`, etc.)
* `form/form_themes.rst` explains everything about form themes, including the
  form fragment naming and all the different options to create themes.

On top of all the above, this PR removes 800 lines of docs!

Commits
-------

7cdf54a Simplified the form theming docs

Author: javiereguiluz

_build/redirection_map

@@ -403,3 +403,5 @@
 /frontend/encore/installation-no-flex /frontend/encore/installation
 /http_cache/form_csrf_caching /security/csrf
 /console/logging /console
+/reference/forms/twig_reference /form/form_customization
+/form/rendering /form/form_customization

_images/form/form-field-parts.svg

@@ -177,8 +177,9 @@ between Twig and several Symfony components:
 
     $ composer require symfony/twig-bridge
 
-The TwigBridge integration provides you with several :doc:`Twig Functions </reference/forms/twig_reference>`
-that help you render the HTML widget, label and error for each field
+The TwigBridge integration provides you with several
+:ref:`Twig Functions <reference-form-twig-functions-variables>`
+that help you render the HTML widget, label, help and errors for each field
 (as well as a few other things). To configure the integration, you'll need
 to bootstrap or access Twig and add the :class:`Symfony\\Bridge\\Twig\\Extension\\FormExtension`::
 
@@ -231,7 +232,7 @@ The exact details of your `Twig Configuration`_ will vary, but the goal is
 always to add the :class:`Symfony\\Bridge\\Twig\\Extension\\FormExtension`
 to Twig, which gives you access to the Twig functions for rendering forms.
 To do this, you first need to create a :class:`Symfony\\Bridge\\Twig\\Form\\TwigRendererEngine`,
-where you define your :ref:`form themes <form-customization-form-themes>`
+where you define your :doc:`form themes </form/form_themes>`
 (i.e. resources/files that define form HTML markup).
 
 For general details on rendering forms, see :doc:`/form/form_customization`.
@@ -510,8 +511,8 @@ Rendering the Form
 
 Now that the form has been created, the next step is to render it. This is
 done by passing a special form "view" object to your template (notice the
-``$form->createView()`` in the controller above) and using a set of form
-helper functions:
+``$form->createView()`` in the controller above) and using a set of
+:ref:`form helper functions <reference-form-twig-functions>`:
 
 .. code-block:: html+twig
 
@@ -528,7 +529,7 @@ That's it! By printing ``form_widget(form)``, each field in the form is
 rendered, along with a label and error message (if there is one). While this is
 convenient, it's not very flexible (yet). Usually, you'll want to render each
 form field individually so you can control how the form looks. You'll learn how
-to do that in the ":doc:`/form/rendering`" section.
+to do that in the :doc:`form customization </form/form_customization>` article.
 
 Changing a Form's Method and Action
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

_images/sources/form/form-field-parts.dia

@@ -90,8 +90,9 @@ shipping options.
 Creating a Template for the Field
 ---------------------------------
 
-Each field type is rendered by a template fragment, which is determined in part by
-the class name of your type. For more information, see :ref:`form-customization-form-themes`.
+Each field type is rendered by a template fragment whose name is determined in
+part by the class name of your type. Read the :ref:`from fragment naming <form-fragment-naming>`
+rules for more details.
 
 .. note::
 
@@ -147,8 +148,8 @@ link for details), create a ``shipping_widget`` block to handle this:
 .. note::
 
     Make sure the correct widget prefix is used. In this example the name should
-    be ``shipping_widget`` (see :ref:`form-customization-form-themes`).
-    Further, the main config file should point to the custom form template
+    be ``shipping_widget`` (see :ref:`form fragment naming <form-fragment-naming>`
+    rules). Further, the main config file should point to the custom form template
     so that it's used when rendering all forms.
 
     When using Twig this is:

components/form.rst

@@ -210,7 +210,7 @@ Override the File Widget Template Fragment
 
 Each field type is rendered by a template fragment. Those template fragments
 can be overridden in order to customize form rendering. For more information,
-you can refer to the :ref:`form-customization-form-themes` article.
+you can refer to the :ref:`form fragment naming <form-fragment-naming>` rules.
 
 In your extension class, you added a new variable (``image_url``), but
 you still need to take advantage of this new variable in your templates.