Integrate Gallery into website's information architecture.

Looking at https://graphviz.org/_pages/Gallery/directed/Genetic_Programming.html, looks like the page was last generated in 2004.

The gallery pages don't have a top nav bar, they're an inconsistent style from the rest of the site.

Bonus points if we can make the gallery a Jekyll collection so we can loop over it and show a list of the gallery pages in a nav sidebar.