This is a sphinx site. You can build it with sphinx directly (assuming you're in a nix-shell):
sphinx-build -j 4 -n . _build
Or you can build it with Nix at the top level, which will also build the Haddock for the project and link it in:
nix build -f default.nix docs.site
The doc site from main is built automatically and hosted here. Additionally, the site is built for all PRs, and a link to a preview can be found in the PR statuses.