I've noticed that the guide from linux from scratch is in portage. So I think the gentoo installation manual (and probably other gentoo docs) should also be in portage Reproducible: Always Steps to Reproduce: 1. 2. 3.
The main issue is that the Gentoo Handbook evolves quite fast; having a script that downloads the latest version is far more efficient than an ebuild imho. As an example, since last week, we had 17 changes.