agentgateway: document best practices for tracking NACKs and provide example

[jmcguire98]

We have some work in flight (metrics, events) to be loud about it when an agentgateway nacks config generated by the kgw control plane. When this work is merged (#12770 is the last outstanding PR) we'll need to work on docs to ensure that we document best practices around this area going forward

[jmcguire98]

Docs have been merged to the OSS kgateway.dev repo 🎉