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.