I have a quickfix for my problem now. It feels very hacky, but I did not find another way.
I now have a shortcode looking like this:
{{ $repo := .Get 0 }}
{{ $json := getJSON "https://api.github.com/repos/europace/" $repo "/readme" }}
{{ $json.content | base64Decode | markdownify }}
Then I call that in my md-file like this:
{{% europace-content "kex-vorgang-import-api" %}}
I am not happy with how it looks, but I am happy that I found something. I will keep this open for a few days as maybe somene has a better solution. Then I will close it