The website is hosted on Github Pages, with the default github.io domain name.
The site sources live in the site branch. For a Github Pages website at the
user or organization level, the built website needs to be pushed on the master
branch.
The site generator is Antora. This website generator can take sources from multiple git repositories. This allows us to keep the documentation for Terraform providers and modules in their respective repositories. On top of that, Antora supports generating documentation for multiple versions of the providers and modules.
If you want a good example of what is achievable with this site generator, have a look at the documentation of Couchbase.