This is the Twitter Open Source website at https://opensource.twitter.dev.
This site is built with hugo, using a custom built-in theme. Follow the standard instructions on the hugo website to install and run hugo. Just make sure to use hugo-extended, which has added support for Sass/SCSS stylesheets.
There are also some python scripts that run periodically to update some repo data.