Consider adding graphile-config-example to docs

### Feature description

I built small node.js project to demonstrate how library authors might use Graphile Config. My hope is that it's good documentation for Graphile Config, and it's easier to look at this repo than to look through one of the larger Graphile projects for how it uses Config.

https://github.com/jcgsville/graphile-config-example

It may make sense to link this from the Graphile Config docs and update the library author docs in Graphile Config. But I imagine Benjie would probably want to review it first to make sure it's sufficiently aligned with how he expects Graphile Worker to be used.

Also open to moving it to graphile-contrib before linking from the docs if you think that makes sense

I'm in no rush here, and I know you're pushing to get v5 out the door, so please don't feel any pressure to move this forward any time soon 🙂

### Supporting development

I'm happy to make the change to the docs if we agree that it would be useful and we agree that it's ready.