Use a neutral example for the gateway endpoint (#14)

The configuration docs named a specific commercial gateway, with its URL, its
key format, a model id from its catalogue and a curl against its endpoint. The
setting works with any OpenAI-compatible endpoint, so the example does not need
a vendor, and naming one reads as an endorsement we did not make.

The substance is unchanged: model names travel verbatim, and a namespaced
catalogue wants both halves of the name.