onboarding: document Red Hat SSO service account login

Miroslav Vadkertirequested to merge
onboarding-service-account-login into main

Document how users can log in to testing-farm.io using a Red Hat SSO service account to create team/group level tokens.

Assisted-by: Claude Code Signed-off-by: Miroslav Vadkerti mvadkert@redhat.com

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