This is the source code for the website of the osm2pgsql software at osm2pgsql.org.
It is written using the static web site generator Jekyll.
To run it locally, you need Ruby. Then check out the repository and install Jekyll:
git checkout https://github.com/openstreetmap/osm2pgsql-website
cd osm2pgsql-website
gem install bundler jekyll
bundle exec jekyll serve
You'll then have a local version running on http://127.0.0.1:4000/.
Read https://osm2pgsql.org/contribute/ for more about helping out.