docs: show how to use an include with a child page

test/default.es.md

@@ -85,9 +85,20 @@ f\colon\left\{\begin{aligned}\mathbb{R}_4[X]&\longrightarrow\mathbb{R}_4[X] \\P&
 > The complex exponential of the circle constant is unity.
 >> $e^{i\tau}=1$
 
-<!--Geogebra animations still to be scaled full window with adapatle height-
+
+
+### Includes
+
+[plugin:content-inject](partials)
+
+
+
+<!--Geogebra animations still to be scaled full window with adapatle height-->
+
 ### GeoGebra
+
 #### Iframe
+
 <!-- https://wiki.geogebra.org/en/Reference:Material_Embedding_(Iframe) -->
 <!--
 <iframe class="geogebra" src="https://www.geogebra.org/material/iframe/id/wkrw5qgm/sfsb/true" allowfullscreen width="320px" height="280px"></iframe>
@@ -111,4 +122,3 @@ See https://wiki.geogebra.org/en/Reference:GeoGebra_Apps_Embedding
 
 <iframe  sandbox="allow-same-origin allow-scripts" src="https://video.samedi.pm/videos/embed/c06dbd9e-d8c7-4655-aade-51ae95b998c3" frameborder="0" allowfullscreen></iframe>
 
-

test/partials/textbook.en.md

+```php
+// Tutorial about PHP
+$course = "Welcome to the PHP course!";
+```