Initially setup via GitHub webinterface and changed/edited markdown pages there. Later switched to a jekyll/CLI usage:
Added a Gemfile with the following content
source "http://rubygems.org"
ruby RUBY_VERSION
# gem "jekyll", "3.4.1"
gem "github-pages", group: :jekyll_plugins
(See pages.github.com/versions for the current dependency versions of gems and more informations about the github-pages gem)
Added a css folder with a main.scss file
bundle exec jekyll serve
``
This will serve the rendered site via http://127.0.0.1:4000/INTPRN/