Improve Chapter 31 SLA lesson

[jules-tth]

Scope

• Reworks Chapter 31.7 into a practical Linux-operator lesson on service level agreements.
• Adds SLA/SLO/SLI vocabulary, measurable SLA areas, Linux evidence examples, maintenance exclusions, error budgets, incident-note guidance, hands-on practice, and review questions.

Verification

• python -m pip install -r requirements.txt failed because python is not on PATH.
• python3 -m pip install -r requirements.txt passed.
• python3 -m mkdocs build --strict passed.
• make build passed.
• git diff --check passed.
• find docs -type f -name '*.md' -size 0 -print returned no files.\n- No dedicated repo-local link-check script was found beyond MkDocs/Makefile build paths.\n\n## Duplicate-prevention\n\n- Open PRs from jules-tth already cover Chapter 31 overview through 31.6.\n- Final pre-PR overlap query returned no open PR touching docs/lessons/31_methodology_policy_and_politics/31.7_service_level_agreements.md.\n

[jules-tth]

Reviewer assignment to bdorlus failed: GraphQL: jules-tth does not have the correct permissions to execute RequestReviewsByLogin (requestReviewsByLogin)