docs: explain metric thresholds and exports

[TristanKruse]

Pull Request

Improve metric guidance so users can choose and interpret architecture thresholds correctly:

• document strict and inclusive comparison semantics
• explain a baseline-first process for selecting thresholds
• summarize how the available metric families should be interpreted
• replace the generic HTML export snippet with a realistic, executable example
• clarify that MetricsExporter formats supplied values while metric rules enforce limits
• mark the corresponding backlog item complete

Change Type

• Bug fix
• Feature
• Documentation
• Refactoring

Validation:

• metric export example executed successfully
• pytest --tb=short -q: 279 passed
• ruff check src tests scripts: passed
• mypy src/archunitpython/ --ignore-missing-imports: passed
• git diff --check: passed