Document saving, loading and distributing snapshots #1976

Snapshots can be saved to and loaded from OCI layouts, but there is no how-to or example covering it. The rustdoc on Snapshot::save and Snapshot::load and docs/snapshot-oci-format.md describe the format, not the workflow.

Add:

• A how-to covering Sandbox::snapshot, Snapshot::save, Snapshot::load and Snapshot::checked_load, building a sandbox from a loaded snapshot, and the file-mutation hazard.
• How to push and pull a snapshot layout with ORAS or another OCI registry client.
• A runnable examples/snapshot program.

Split from #1479.