Docs can be found in the flarum/docs repo.
You can edit small changes online using the GitHub editor. For larger edits it is recommended to make a local checkout and test your changes before you commit them. To do that, you need Ruby. Instructions for Linux/Mac:
git clone https://github.com/flarum/flarum.github.io cd flarum.github.io cat > Gemfile <<EOM source 'https://rubygems.org' gem 'github-pages', group: :jekyll_plugins EOM bundle install bundle exec jekyll serve
Now, you can view your changes through http://localhost:4000. Jekyll will automatically rebuild the site when you save your edits.