Add a job creating a release tarball to GitLab CI

Add a job creating a release tarball to GitLab CI

Add a GitLab CI job (which is run only if all other jobs in a pipeline
succeed) that builds a BIND release tarball, i.e. fetches the source
tarball from the tarball building job, creates Windows zips, puts
certain parts of BIND documentation into the appropriate places, and
packs it all up into a single tarball whose contents can be subsequently
signed and published.