docs(ansible): update nvidia-container-toolkit manual installation steps

[xmfcx]

Summary

• Related: Support ROS 2 Jazzy Jalisco #6695

This PR updates the manual installation instructions for the NVIDIA Container Toolkit to reflect the current recommended upstream procedure. The documentation now uses modern APT key handling, updated prerequisites, and a newer CUDA test container.

⚠️ These steps are explicitly marked as recommended manual installation and may differ from the current Ansible role implementation. The role will be aligned in a follow-up change.

Key Changes

• Clarified installation intent

  • Renamed Manual Installation → Manual Installation (Recommended).
  • Added a note explaining the temporary divergence from the Ansible role.

• Modernized repository setup

  • Replaced deprecated apt-key usage with gpg --dearmor and a signed APT source.
  • Added explicit prerequisites (curl, gnupg2).

• Updated verification example

  • Switched to a newer CUDA base image:

    • From cuda:12.3.1-runtime-ubuntu20.04
    • To cuda:12.8.1-runtime-ubuntu24.04

  • Updated sample nvidia-smi output accordingly.

Motivation

• Align documentation with NVIDIA’s current best practices.
• Avoid deprecated APT workflows that may fail on newer systems.
• Provide a clearer, copy-paste–safe manual setup for users troubleshooting GPU/container issues.

Testing

• Documentation-only change.
• Manual steps validated against current NVIDIA Container Toolkit instructions.

Notes for Reviewers

• The Ansible role still uses older logic; this PR intentionally documents the preferred approach first.
• A follow-up PR will update the role implementation to match these steps.

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