update docs for debian testing and unstable
update docs for debian testing and unstable

Buildah is now present on debian testing and unstable so providing Kubic
repos for it can lead to conflicts. Better to mention only the official
repos to avoid confusing end users.
Related: https://github.com/containers/buildah/issues/2797

Also, provide installation steps for Ubuntu 20.10 and newer, and redirect
Raspberry Pi OS users to Debian installation steps to
avoid content duplication.

Signed-off-by: Lokesh Mandvekar <46b23e80209bb43857d25fd87bafe9be89c7d33a@fedoraproject.org>

nalind/buildah-1,rhatdan/buildah,nalind/buildah-1,TomSweeneyRedHat/buildah,TomSweeneyRedHat/buildah,nalind/buildah-1,nalind/buildah,projectatomic/buildah,rhatdan/buildah,projectatomic/buildah,TomSweeneyRedHat/buildah,projectatomic/buildah,rhatdan/buildah,nalind/buildah