add information on monorepo docs for service role creation

[ykethan]

Describe the content issue:
A clear and concise description of what the content issue is. Is there content missing or is there incorrect content?

when deploying a frontend only app by default the service role is not attached but npx ampx generate outputs --branch <backend-app-branch> --app-id <backend-app-id> requires a service role to be attached for this to work.
adding a callout on attaching a role with backend deploy policy to the app would be useful

ref issue: aws-amplify/amplify-backend#2321

URL page where content issue is:
https://docs.amplify.aws/react/deploy-and-host/fullstack-branching/mono-and-multi-repos/#deploy-the-frontend-app
https://docs.amplify.aws/react/deploy-and-host/fullstack-branching/mono-and-multi-repos/