You will need to have Maven installed. You can build the project by issuing mvn clean verify.
When the build finishes you will find an installable Eclipse repository in the repository/org.jboss.tools.quarkus.site/target folder.
The name of the repository is org.jboss.tools.quarkus.site-x.y.z-SNAPSHOT.zip.
Now you can proceed with the installation of your repository in your Eclipse workbench by following the installation instructions.