Updated offline documentation

[bbaassssiiee]

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespaces from that line:

/kind api-change
/kind bug
/kind cleanup
/kind design
/kind documentation
/kind failing-test
/kind feature
/kind flake

What this PR does / why we need it:
This is an update to the offline documentation that explains using private registries with containerd 2 or 1.7. It also reduces the number of URLs with the suggestion to use the original domains as top-registries in the files_repo.

Which issue(s) this PR fixes:

Fixes:

• containerd_registry_auth not working #12664
• v2.28.1 Containerd 2.0: It's not possible to configure private repository with authentication #12575

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Offline files repo is expected to use original domains as top-level directories.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: bbaassssiiee
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[k8s-ci-robot]

Hi @bbaassssiiee. Thanks for your PR.

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[tico88612]

Quick look

[yankay]

/ok-to-test

[yankay]

HI @ErikJiang

Would you please help to review it.