Improve Chapter 01 suggested background lesson

[jules-tth]

Summary

• replace the generic suggested-background page with a practical Linux readiness guide
• clarify which computer, CLI, editing, networking, scripting, and Git skills help before deeper lessons
• add a safe-lab practice flow and review questions focused on evidence and recoverability

Verification

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

Reviewer assignment note: previous automated reviewer requests from this repo failed for this token with RequestReviewsByLogin permissions, so this PR body records the review request for @bdorlus.