[doc] Add requirement for gettext and graphviz
When running the tutorial from scratch, I had errors because these
two packages were not installed.
extend-diff-ignore = "^(__pkginfo__\.py$|doc/_themes/cubicweb/static/(logo-cubicweb-small\.svg|cubicweb\.ico|logo-cubicweb\.svg)$|cubicweb\.spec$|setup\.cfg$|cubicweb/misc/cwfs)"