dgit-user(7): Suggest sbuild-debian-developer-setup(1)

dgit-user(7): Suggest sbuild-debian-developer-setup(1)

This will only create the chroot for building packages for the
machine's architecture, because sbuild-debian-developer-setup(1) does
not have an --architecture option, so sbuild-createchroot(1) will be
needed too, hence the 'and'.

The nice thing about sbuild-debian-developer-setup(1) is that does
fiddly things that a non-expert might forget about, like adding the
current user to the sbuild group.  So it is still worth recommending
even though it does not have an --architecture option.

Closes:#895779.

Signed-off-by: Sean Whitton <spwhitton@spwhitton.name>