minor #778 React components : working twig example (hedocode)

This PR was squashed before being merged into the 2.x branch.

Discussion
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React components : working twig example

Hello ! The tutorial didn't work because I had not the following lines in my twig :

{% block stylesheets %}{{ encore_entry_link_tags('app') }}{% endblock %}
{% block javascripts %}{{ encore_entry_script_tags('app') }}{% endblock %}

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| Tickets       | N/A <!-- prefix each issue number with "Fix #", no need to create an issue if none exist, explain below instead -->
| License       | MIT

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Commits
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a7e8ae5 React components : working twig example

Author: weaverryan

src/React/doc/index.rst

@@ -62,7 +62,7 @@ Finally, to load your React components, add the following lines to ``assets/app.
     // they are not necessary.
     registerReactControllerComponents(require.context('./react/controllers', true, /\.(j|t)sx?$/));
 
-That's it! Create an `assets/react/controllers/` directory and start creating your
+That's it! Create an ``assets/react/controllers/`` directory and start creating your
 React components.
 
 Usage
@@ -77,6 +77,7 @@ React components located in the directory ``assets/react/controllers`` are regis
 React controller components.
 
 You can then render any React controller component in Twig using the ``react_component``.
+
 For example:
 
 .. code-block:: javascript
@@ -88,11 +89,19 @@ For example:
         return <div>Hello {props.fullName}</div>;
     }
 
-
 .. code-block:: html+twig
 
     {# templates/home.html.twig #}
-    <div {{ react_component('MyComponent', { 'fullName': app.user.fullName }) }}></div>
+    {% extends 'base.html.twig' %}
+
+    {% block body %}
+        <div {{ react_component('MyComponent', { 'fullName': number }) }}>
+            Loading... <i class="fas fa-cog fa-spin fa-3x"></i>
+        </div>
+        
+        {# Component living in a subdirectory: "assets/react/controllers/Admin/OtherComponent" #}
+        <div {{ react_component('Admin/OtherComponent') }}></div>
+    {% endblock %}
 
 Backward Compatibility promise
 ------------------------------