Update README.md to streamline airgap artifact preparation using seac…

[alknopfler]

This pull request updates the instructions in the telco-examples/edge-clusters/airgap/README.md for preparing airgap artifacts. The new approach replaces a multi-step, script-based process with a streamlined workflow using the seactl CLI tool embedded in the SUSE Edge release container image. The documentation now guides users through using seactl for all artifact mirroring tasks, including authentication setup, and provides updated commands and notes for best practices.

Streamlined airgap artifact preparation using seactl:

• Replaces the previous multi-script process for mirroring RKE2 images, Helm charts, and container images with a single seactl command, simplifying the workflow and reducing manual steps.
• Adds instructions for creating registry authentication files (including for Rancher Apps and SUSE Private Registry) to support authenticated mirroring with seactl.
• Provides updated example commands for running the release container with podman, including bind-mounting credentials and certificates, and demonstrates both standard and SUSE Private Registry scenarios.
• Updates the artifact copying step to reflect the new output directory structure generated by seactl.
• Includes a note clarifying that the release container image already contains the necessary manifest files, and