Add a link to the Example Gallery from the Forum Example

Replaces https://github.com/graphile/postgraphile/issues/874

Locate the Forum Example in the graphile.github.io website content and inspect how the Example Gallery is linked elsewhere. Add the Gallery link to that example, then verify the rendered page contains the link and that it points to the intended gallery.

索引モデルが issue の本文から書いたものです。