docs: add rule/metric authoring guide (#25)

[TamasBoncz]

This pull request introduces comprehensive documentation for authoring detection rules and metrics in AI Engineer Coach. It adds a new authoring guide and updates the contributing guidelines to help users create, test, and contribute rules and metrics without code changes.

Closes #25

Documentation improvements:

• Added a new docs/AUTHORING_RULES.md guide that explains the file format, DSL, testing workflow, and provides annotated examples for both rules and metrics.
• Updated CONTRIBUTING.md with a section on creating rules and metrics, including pointers to the new authoring guide and relevant source directories.## Description

Related Issues

Checklist

• npm run check passes (typecheck + lint + spellcheck + knip + tests)
• Changes are covered by tests (if applicable)
• Documentation updated (if applicable)

[github-actions]

⚠️ Deprecation Warning: The deny-licenses option is deprecated for possible removal in the next major release. For more information, see issue 997.

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

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