Hi Perlkönig,
I'm starting at the grav and did not know how to start, but I found a post[1] that indicates to install the "taxonomylist" plugin and add the following snippet at the end of the code that renders the navigation menu:
navigation.html.twig
{% set taxlist = taxonomylist.get() %}
{% if taxlist %}
{% set category = grav.uri.params("category", true) %}
{% for tax,value in taxlist['category'] %}
{% set current_page = (tax == category) ? 'active' : '' %}
<li class="{{ current_page }}">
<a href="{{ base_url }}/category{{ config.system.param_sep }},{{ tax|e('url') }}">{{ tax|capitalize }}</a>
</li>
{% endfor %}
{% endif %}
It works, but need a few adjustments. ;)
[1] http://diblas.net/blog/add-the-posts-categories-to-the-blog-navigation-menu-powered-by-grav-cms