Found the mistake - Markdown content should reside in .md files :).

_config.yml

@@ -21,8 +21,8 @@ feed:
   de: /flux.xml
 baseurl: "/help" # the subpath of your site, e.g. /blog
 url: "https://jlugitlab.gitlab.io" # the base hostname & protocol for your site
-# twitter_username: jekyllrb
-# github_username:  jekyll
+twitter_username: false
+github_username: false
 
 # Build settings
 markdown: kramdown

angaben.html → angaben.md

 ---
 layout: page
-title: About
+title: Angaben
+permalink: /angaben/
 ref: about
 lang: de
 ---
 
-This is the base Jekyll theme. You can find out more info about customizing your Jekyll theme, as well as basic Jekyll usage documentation at [jekyllrb.com](http://jekyllrb.com/)
+Dies ist der Jekyll Basisstil. Mehr Informationen zum Anpassen deines Jekyll-Stils, sowie erste Schritte wie man Jekyll benutzt findest Du auf [jekyllrb.com](http://jekyllrb.com/)
 
-You can find this multilanguage theme at
+Du findest diesen Mehrsprachigen Stil auf
 {% include icon-github.html username="sylvaindurand" %} /
 [multilingual-jekyll](https://github.com/sylvaindurand/multilingual-jekyll)
 
-You can find the original source code for the Jekyll new theme at
+Du findest den originalen Quellcode des Stils "Jekyll new" auf
 {% include icon-github.html username="jglovier" %} /
 [jekyll-new](https://github.com/jglovier/jekyll-new)
 
-You can find the source code for Jekyll at
+Du findest den Quellcode für Jekyll auf
 {% include icon-github.html username="jekyll" %} /
 [jekyll](https://github.com/jekyll/jekyll)