Explicit the packages that are not official

[beta-ziliani]

Those packages that are not maintained by us should loudly say they aren't.

[straight-shoota]

Ref: https://github.com/crystal-lang/crystal-website/pull/472#issuecomment-1642019936

Sources that are not owned by us are distribution package repositories like BSD ports and packages, apk (Alpine), portage (Gentoo), pacman (Arch Linux). And most repos for 3rd party package managers: asdf, homebrew, scoop, termux, nix.

We can add notes that they are not official and may not be up to date.

Actually, nix is not listed anywhere. But it should. 🚀

[HertzDevil]

FYI there are even more unofficial packages: