Update README with Longhorn and system extensions documentation

[winlinuxmatt]

Summary

This PR updates the README to comprehensively document the Longhorn persistent storage setup and Talos system extensions configuration.

Changes

New Sections Added

• Talos System Extensions: Documents the custom Talos image built via Image Factory with:

  • iscsi-tools v0.2.0
  • util-linux-tools 2.41.1
  • qemu-guest-agent 10.0.2
  • Schematic ID and verification commands

• Persistent Storage with Longhorn: Complete guide including:

  • Feature overview (distributed storage, dynamic provisioning, web UI, backups)
  • Helm installation instructions
  • Pod security configuration steps
  • Verification commands for pods and storage classes
  • UI access information (NodePort 30080)
  • Link to detailed LONGHORN_TALOS_SETUP.md

Updated Sections

• Key Features: Added custom Talos image and Longhorn storage highlights
• Directory Structure: Updated to include:
  • longhorn-values.yaml
  • LONGHORN_TALOS_SETUP.md
  • templates/ directory with worker-disks.yaml.tmpl and cpnetwork.yaml.tmpl
• Use talosctl: Added system extensions verification example

Documentation Improvements

• Clear step-by-step instructions for Longhorn deployment
• Verification commands with expected output examples
• Cross-references to detailed setup guide
• Complete picture of cluster capabilities and configuration

This documentation reflects all changes from PR #25 (now merged) and provides users with a complete understanding of the cluster setup.