docs: retire OpenShell Community repository

[johntmyers]

Summary

Replace the catalog-oriented README with a retirement notice and migration guidance before this repository is archived. The new guidance directs users to maintain workload images, policies, and provider profiles in repositories they control and shows how to launch that artifact set with current OpenShell commands.

Related Issue

No issue required: this is a documentation-only repository retirement notice.

Changes

• Mark the repository and its published artifacts as retired and unsupported
• Document that current OpenShell defaults to nvcr.io/nvidia/base/ubuntu:24.04 and no longer expands Community catalog aliases
• Recommend a self-owned repository layout for OCI images, policies, and provider profiles
• Add a current build, provider import, and sandbox launch workflow
• Redirect documentation, support, and security reporting to NVIDIA/OpenShell

Testing

• python3 scripts/check_license_headers.py --check passes
• git diff --check passes
• All external README links return HTTP 200
• mise run pre-commit (this repository defines no mise tasks)

Checklist

• Follows Conventional Commits
• Commit is signed off (DCO)