Improve CSS build process

Currently we are always loading the css file from /dist/css/site.min.css:

<link rel="stylesheet" href="/user/themes/knotentheme/dist/css/site.min.css">

Thereby we are bypassing the Grav Asset Manager, which is loading the stylesheet the following way:

    {% block stylesheets %}
        {% do assets.addCss('theme://dist/css/site' ~ extension ~ '.css', 98) %}
    {% endblock %}

This has the following disadvantages:

the npm run watch command doesn't work for local development, because it only creates a site.css-file
we had to disable file caching for css files and loosing the versioned asset file names from the default Grav Asset Manager

metaknoten / knotentheme