Improve Chapter 06 software overview

[jules-tth]

Summary

• Reframe Chapter 06 around traceable software installation and package-management operations
• Add concrete package/source inspection commands and a practical change-discipline checklist
• Preserve the lesson index while cleaning up lesson titles

Verification

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

Duplicate-prevention

• Full open-PR coverage check found this file uncovered after PR Improve Chapter 05 filesystem overview #313.
• Final target-file overlap query returned no open PR touching docs/lessons/06_software_installation_and_management/README.md immediately before PR creation.

Reviewer assignment note: gh may not have permission to request bdorlus as reviewer from this runtime; BobbyD remains the intended reviewer.