Is this supported? Is there a manual on how to do so?
Yes, it is possible to create a site-to-site VPN using ZeroTier on a USG, but you will have to use ZeroTier 1.4.6 or build from source. HERE is a link to the knowledge base entry for Ubiquiti routers. It does not explain how to set up site-to-site, but that can be found in other places in the knowledge base. This article can be used to get the basics, and then some basic networking knowledge is needed to get the routing set up correctly.
You also have to setup the routing, I did it with a USG edge router and OPNsense.