Site should canonically live on graphviz.org, not graphviz.gitlab.io.

The graphviz.gitlab.io is tied to Graphviz's current hosting provider, which might change.

graphviz.org is a 'forever home' not tied to a provider.

However on https://graphviz.org, I see <link rel="canonical" href="https://graphviz.gitlab.io/"> instructing search engines that the canonical copy of this site is up graphviz.gitlab.io, and that's where we should send users. I propose inverting this.