Add examples of building a kubert controller with Gateway API

[shaneutt]

We need an example in gateway-api/examples for building a kubert controller with Gateway API.

[shaneutt]

Kubert is great, but since it's not part of a CNCF effort there's a little less certainty around it overall. Still open to having an example in the repository, but closing this as not planned just to signal that it's not a prioritized thing.