Add Graphviz to Docsy theme example list
Add Graphviz to Docsy theme example list

I migrated the Graphviz docs to Docsy today with https://gitlab.com/graphviz/graphviz.gitlab.io/-/merge_requests/353.

Thanks for Docsy, it's great being able to outsource a lot of the hard parts of technical documentation to a central team.
google/docsy,grpc/grpc.io-docsy,google/docsy,grpc/grpc.io-docsy