save: add --quiet/-q to suppress progress output

[s3onghyun]

Fixes #4958.

Re-opening #5054, which I accidentally closed with a force-push. This is the same single squashed commit (expect.ExitCodeSuccess review suggestion from @sathiraumesh already applied). Thanks @AkihiroSuda.

nerdctl save always writes a progress bar to stderr, which clutters logs when it is used in scripts and CI. This adds a --quiet/-q flag that suppresses the progress output.

The implementation mirrors the existing --quiet flag on nerdctl load: the flag is threaded into ImageSaveOptions.Quiet, and when set, the transfer progress handler writes to io.Discard instead of os.Stderr. The archive itself is written exactly as before; only the progress output is suppressed.

• cmd/nerdctl/image/image_save.go: register -q, --quiet and pass it into the options
• pkg/api/types/image_types.go: add Quiet to ImageSaveOptions
• pkg/cmd/image/save.go: route progress to io.Discard when quiet
• docs/command-reference.md: document the flag
• cmd/nerdctl/image/image_save_test.go: add TestSaveQuiet verifying the archive is still produced correctly with --quiet

Manual check: nerdctl save --help now lists -q, --quiet Suppress the progress output.