Devuan instructions

Our instructions for Debian and Devuan are identical except the URL for backports is different. Do we really need separate instructions and dropdowns for the two systems? I added them after my conversation at https://github.com/certbot/certbot/pull/4903, however, I personally think a better path forward is to remove "Debian (other") and all Devuan instructions. We can add a note to the Debian instructions about the backports URL if we want, however, I suspect most Devuan users will be able to figure this out on their own.

If we don't want to do this and we still want explicit instructions for Devuan, I think we need to add new dropdowns for Devuan like we do for Debian to explain what's available on each system.