Extend resource validation doc to cover NKG CRDs

[pleshakov]

https://github.com/nginxinc/nginx-kubernetes-gateway/blob/c7e05f4f404ceaad743d2681940d52a59a34d19b/docs/resource-validation.md describes how NKG validates Gateway API resources.

Because NKG now introduced a CRD, it makes sense to extend that doc to describe how NKG validates NKG resources.

Acceptance

• The doc is updated to explain that NKG resources, specifically our control plane CRD, is validated.