So any explanation/links/pointers would be appreciated here. (Also, AFAIK, this has never been used before in any other Fedora project – or CoreOS, has it?)
Ignition is new to the Fedora ecosystem, but has been offered in Container Linux since 2015. Ignition’s introductory material and documentation index are available. Note that Ignition configs are not intended to be written by hand; they’re typically generated from a Container Linux Config. (For Fedora CoreOS, the latter will evolve into something like a “Fedora CoreOS Config”).
Also Ignition is the best thing ever when used with the dedicated Terraform plugin, best way to configure a server declaratively. So it’s very nice that this is kept from CoreOS.