The Terraform stuff is in _infrastructure/ as anything starting with _* will be hidden by Jekyll and it feels like the right place to put it.
https://help.github.com/articles/files-that-start-with-an-underscore-are-missing/
By default, Jekyll does not build any files or directories that
- are hidden or used for backup (indicated by names that start with . or #, or that end with ~);
- contain site content (indicated by names that start with _); or
- are excluded in the site configuration.