add information on monorepo docs for service role creation

**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 --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: https://github.com/aws-amplify/amplify-backend/issues/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/

Open the linked mono-and-multi-repos documentation pages and start at the “Deploy the frontend app” section. Check the existing instructions around `npx ampx generate outputs --branch --app-id `. Done means adding a callout explaining that a service role with the backend deploy policy must be attached before running the command.

Written by the indexing model from the issue text.