Document how to configure sandbox build jobs to host artifacts

Typically RPM/Deb build jobs push packages to Nexus for hosting. However, jobs running in the sandbox shouldn't push under-dev packages to Nexus, and don't have permission to do so. However, it's still nice to be able to build a test RPM on the Sandbox and link to it, or consume it in other tests later, or generally access it at all. Best option seems to be to manually add a post-build step to archive artifact in `sandbox/<job>/configure` section on web UI. Doc this process on the packaging RTDs.

Imported from Trello