Documentation updates for Argo Rollouts tutorial

[jastang]

Adds instructions for forking the demo repository and alternatively registering the app with ArgoCD.

Should at least address some of the comments in xref #501.

[jastang]

@kflynn @JWKartheiser tagging as requested on Slack (I can't add reviewers to PRs)

I'll open a separate issue re: CODEOWNERS config. Thanks!