Update Notification services docs

[RaviHari]

Summary

Argo Rollout Notification services documentation uses app in the templates instead of rollouts as it might have been taken from argocd-notifications documentation.
https://argoproj.github.io/argo-rollouts/generated/notification-services/email/
It would be good to update it with rollout templates in all the services documentaitons.

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[meeech]

To help anyone coming later - the bulk of the notif docs seem to be pulled in: https://github.com/argoproj/argo-rollouts/blob/master/hack/gen-docs/main.go#L28

[meeech]

Docs should give guidance to the namespace that the notifications-install.yaml should be installed to: same namespace as the controller

[github-actions]

This issue is stale because it has been open 60 days with no activity.

[kzap]

I was looking at this in my PR #3384 but its hard to keep up to date with changes in the docs if they are specific to ArgoCD, I would prefer if the notifications-engine docs were project agnostic and each project would make their replacements

Although a string replacement is simple, entire examples may not be...