Add fallback guide for Packit Service jobs (#1141)

Add fallback guide for Packit Service jobs

Documents how to manually reproduce each automated Packit Service job (copr_build, tests, propose_downstream, pull_from_upstream, koji_build, bodhi_update, upstream_koji_build, vm_image_build) with the Packit CLI or the underlying tooling, so maintainers are not blocked when the service is degraded. Addresses packit/packit-service#1657.
Assisted-By: Claude Opus 4.7 (1M context) noreply@anthropic.com

Reviewed-by: Tomas Tomecek <tomas@tomecek.net>
Reviewed-by: Nikola Forró
Reviewed-by: Maja Massarini
Reviewed-by: Laura Barcziová

Author: centosinfra-prod-github-app[bot]