Expand course glossary operator terms

[jules-tth]

Scope

• Expanded docs/course/glossary.md from a thin starter list into a practical operator glossary.
• Added concise definitions for core course concepts across boot, networking, storage, DNS, process control, security, systemd, CI/CD, and operations practice.
• Kept the change isolated to the Course glossary page to avoid overlapping the pending chapter PRs.

Duplicate-prevention check

• Open PRs from jules-tth already cover lesson files through Chapter 31 and docs/course/syllabus.md.
• Final target-file overlap query returned no open PR touching docs/course/glossary.md.

Verification

• python -m pip install -r requirements.txt could not run 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.

[jules-tth]

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