Improve Chapter 01 units lesson

[jules-tth]

Summary

• Rewrite the Chapter 01 units lesson around practical Linux operations
• Clarify bytes vs bits, decimal vs binary units, and common command output
• Add a safe lab and incident-handoff oriented checks

Verification

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

Duplicate prevention

• Checked open PRs for 1.06_units immediately before commit and PR creation; none were open for this file.
• Found existing PR Improve Chapter 01 notation conventions lesson #304 for the prior planned 1.05 slice and skipped it.