Improve Chapter 01 sysadmin duties lesson

[jules-tth]

Scope

• Reworks Chapter 01's first duties lesson into a practical Linux sysadmin responsibility map.
• Adds read-only baseline commands, routine-vs-risky work guidance, production safety warnings, a first-week workflow, troubleshooting signals, lab practice, and review questions.
• Keeps the change scoped to docs/lessons/01_where_to_start/1.01_essential_duties_of_a_system_administrator.md.

Duplicate prevention

• Checked open PRs from jules-tth; PR Improve Chapter 01 orientation #3 touches only the Chapter 01 overview README.
• Re-ran a target-file overlap query immediately before PR creation; no open PR touched this lesson file.

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: passed, no files returned.
• Repo-local check script scan found no dedicated link/check script beyond MkDocs/Makefile build paths.

[jules-tth]

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