Add deployment manifests as a release artifact

[ciarams87]

As a potential user of NKG
I want the manifest files available as a release artifact
So that when I install/ upgrade/ uninstall NKG using manifests, I don't have to clone the repo

Ideally, a user would be able to install NKG by doing something like:

kubectl apply -f https://github.com/nginxinc/nginx-kubernetes-gateway/releases/download/v0.6.0/nginx-kubernetes.yaml

Acceptance

• Update the pipeline to add the generated yaml installation manifest as a release artifact
• Ensure the artifact is signed (if appropriate)
• The service manifest is NOT uploaded as an artifact.
• Update the yaml installation documentation and include instructions on creating a service to work with NKG.

[kate-osborn]

This might be a good issue for @bjee19 to pair on to learn about GitHub actions and release artifacts.