Update resource validation design doc with strategy how to validate Gateway API and NGF resources

[pleshakov]

Acceptance criteria:

• Update resource validation design doc with strategy how to validate Gateway API and NGF resources

Needed for #1984 and future development work for implementing Gateway API resources and new NGF resources.

https://github.com/nginxinc/nginx-gateway-fabric/blob/main/design/resource-validation.md