Improve Chapter 05 filesystem overview

[jules-tth]

Summary

• Reframe Chapter 05 overview around filesystem evidence and operator troubleshooting
• Add concrete read-only inspection commands for path, mount, capacity, permission, and ACL triage
• Preserve the lesson index while cleaning up titles and removing motivational filler

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 no open PR touching docs/lessons/05_the_filesystem/README.md immediately before creation.

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