Improve Chapter 01 Linux distributions lesson

[jules-tth]

Summary

• Rework the Linux distributions lesson into a practical operator guide
• Cover distribution families, release models, package sources, support windows, and safety checks
• Add a distribution inventory lab and review questions

Verification

• python3 -m mkdocs build --strict
• git diff --check
• find docs -type f -name '*.md' -size 0 -print
• make build

[jules-tth]

Closing as a duplicate of #301, which already covers docs/lessons/01_where_to_start/1.03_linux_distributions.md and has passing CI.