Add default salsa-ci/pipeline config

I'm not really sure how useful the default pipeline is for debian-cd, since that doesn't really test if things work in the way they are normally used, but just checks the packaging, but at least having this enabled in the main repo means that we have a starting point from which to add jobs that do more useful tests, such as attempting to build some ISOs to see if that still works, or perhaps a little extra motivation to add autopkgtests, say.

Anyway, I enabled this in my fork, and it gives green ticks for everything, so I thought I'd fire off an MR :-)

BTW In order for this to work, one needs to set the project's: CI/CD > General pipelines > CI/CD configuration file setting to: debian/salsa-ci.yml

UPDATE: read my comment below about getting the same effect without adding a file to the repo.