[html] adjust doku to include twig

[astroidex]

Describe the bug

• https://doc.mapbender.org/de/elements/misc/html.html#verweis-auf-eine-twig-datei
• new syntax for Mapbender 4
• {% include 'news.html.twig' %}
• place files at application/Templates

news.html.twig
{% include 'news.html' %}

news.html

     <p><b>Mapbender</b></p>
     <p>Mapbender is an open source WebGIS solution.</p>
     <p>Read more at <a href="https://mapbender.org/" target="_blank">https://mapbender.org</a>

Try to make an example with {% include 'WorkshopDemoBundle@news.html.twig' %}