doc is no longer shipped as part of the rsyslog tarball

Instead, the rsyslog-doc project creates its own tarball. This is the
result of a mailing list discussion after the 8.2.0 release with a
tarball-in-tarball approach, which was disliked by almost all distro
maintainers. This move also has the advantage of de-coupling the
release cycles of both projects a bit (which turned out to be a bit
problematic in practice).