Added the JS and CSS assets required by the doc theme

Author: javiereguiluz

_build/_theme/_templates/layout.html

@@ -1,13 +1,15 @@
 {% extends '!layout.html' %}
 
-{% set css_files = ['https://symfony.com/css/compiled/v5/all.css?v=4'] %}
+{% set css_files = ['./assets/css/app.css', './assets/css/doc.css'] %}
 {# make sure the Sphinx stylesheet isn't loaded #}
 {% set style = '' %}
 {% set isIndex = pagename is index %}
 
 {% block extrahead %}
 {# add JS to support tabs #}
-<script src="https://symfony.com/js/v5/all.js?v=4"></script>
+<script src="./assets/js/manifest.js"></script>
+<script src="./assets/js/app.js"></script>
+<script src="./assets/js/doc.js"></script>
 
 {# pygment's styles are still loaded, undo some unwanted styles #}
 <style>