docs: fix intersphinx and add automatic maintenance

Without this sphinx won't do the necessary linkage for documentation references back into these projects. See https://github.com/pkgcore/pkgcore/actions/runs/7656999844/job/20874145430 for what kicked all of this off. I've not checked pkgdev docs to see if this linkage is explicitly used, but the sphinx module *is* used, thus adding this.