The Ruffle website uses Jekyll. It is visible at https://ruffle.rs/
To test it locally, you should clone the repository (git clone https://github.com/ruffle-rs/ruffle-rs.github.io.git). If you plan to create a pull request, you should instead fork the repository, then clone the forked repository. Either way, you should then install all the pre-requisites for your system as described in https://jekyllrb.com/docs/installation/.
Then, navigate to the directory where you cloned the repository and run:
gem install jekyll bundlerbundle installbundle exec jekyll serve
If everything worked properly, a copy of the website should be up and running at http://localhost:4000 and any changes you make to the code should appear there. Any time you want to test changes to the website after that, re-run bundle exec jekyll serve. If you are told you are missing gems when bundling, re-run bundle install and then re-run bundle exec jekyll serve.