Add help_topics example

Thank you for providing a merge request!

The issue is not about adding hook_help() implementations, though. It is about adding help topics, which are provided using template files, for example with the following content.

---
label: 'Extending and modifying your site functionality'
top_level: true
---
<h2>{% trans %}What is a module?{% endtrans %}</h2>
<p>{% trans %}A <em>module</em> is a set of PHP, JavaScript, and/or CSS files that extends site features and adds functionality. A set of <em>Core modules</em> is distributed as part of the core software download. Additional <em>Contributed modules</em> can be downloaded separately from the <a href="https://www.drupal.org/project/project_module">Download &amp; Extend page on drupal.org</a>.{% endtrans %}</p>
<h2>{% trans %}What is an Experimental module?{% endtrans %}</h2>
<p>{% trans %}An <em>Experimental</em> module is a module that is still in development and is not yet stable. Using Experimental modules on production sites is not recommended.{% endtrans %}</p>
<h2>{% trans %}What are installing and uninstalling?{% endtrans %}</h2>
<p>{% trans %}Installing a core or downloaded contributed module means turning it on, so that you can use its features and functionality. Uninstalling means turning it off and removing all of its configuration. A module cannot be uninstalled if another installed module depends on it, or if you have created content on your site using the module -- you would need to delete the content and uninstall dependent modules first.{% endtrans %}</p>
<h2>{% trans %}Extending overview{% endtrans %}</h2>
<p>{% trans %}See the related topics listed below for help performing tasks related to extending the functionality of your site.{% endtrans %}</p>
<h2>{% trans %}Additional resources{% endtrans %}</h2>
<ul>
    <li>{% trans %}<a href="https://www.drupal.org/docs/user_guide/en/understanding-modules.html">Concept: Modules (Drupal User Guide)</a>{% endtrans %}</li>
    <li>{% trans %}<a href="https://www.drupal.org/docs/user_guide/en/extend-chapter.html">Extending and Customizing Your Site (Drupal User Guide)</a>{% endtrans %}</li>
</ul>