docs: clarify current OS validation targets

[dholt]

Summary

• Clarify which operating-system paths are current release validation targets versus retained legacy/community references.
• Update current docs and examples to emphasize Ubuntu 22.04/24.04, DGX OS 6/7, and RHEL/Rocky 8/9 DGX software-stack coverage where applicable.
• Fix the NGC Ready playbook path to playbooks/ngc-ready-server.yml and label older EL/CentOS task paths as legacy.

Validation

• git diff --check origin/master..HEAD
• Focused syntax check for playbooks/bootstrap/bootstrap-python.yml and playbooks/ngc-ready-server.yml
• OS compatibility audit against this branch: medium=5, low=67, info=29
• Public sanitizer check on the PR body and added diff lines

Notes

• This is a documentation/support-boundary cleanup. It does not remove the legacy playbooks or roles.
• The OS compatibility audit still reports medium findings for workload and RoCE follow-up work, which is queued for later stacked branches.