As agreed in tracker#39, we're
going to be maintaining the treefile for all Fedora versions in the
`main` branch. Accordingly stop hardcoding releasevers and repos in
the treefiles.

But for now, we're still building the official image in pungi using the
non-container-native path, so add compatibility stubs to not break it.

Coincidentally, this doubles down on the proposed API to build the base
images for different Fedora versions using the `--from` switch. Document
this in the README. But note this doesn't work yet because nothing is
telling rpm-ostree which repos to use or what the releasever should be.