Add new hugo template for www.system-transparency.org

The current ST website sends a few red flags, like having outdated content and being heavy to render. As discussed before, we would like to fix this.

We would like the "fix" to not turn into a huge project.

A concrete plan to move this forward could be: do a copy-paste based on Sigsum's website.

Step 1 (this issue): fix hugo template so that it works for ST's site.