We use Jekyll, a static site generator written in Ruby. See Quickstart for GitHub Pages for a series of introductory tutorials.
In short, install Bundler, a tool for managing Ruby gems, and then run the following shell commands.
# Set the location on disk where all gems in the bundle will be located.
bundle config set path '.bundle/vendor'
# Install the gems in Gemfile.
bundle install
# Serve the website with incremental regeneration for pages that were updated since the previous build.
bundle exec jekyll serve --incremental