Address discrepancy between docs and reality

[ahpook]

The documentation used to say that you needed to transform keys in external config files from using - to _, but in reality the code transforms - to _ regardless of where they occur.

See 4b4ec08

Closes #909

[Copilot]

Pull Request Overview

This PR addresses a documentation inconsistency by correcting the external configuration file format to use dashes instead of underscores, aligning with the actual code behavior. The documentation previously incorrectly stated that external config files needed underscores, when the code actually transforms dashes to underscores automatically.

• Updated table of contents to include additional section links
• Corrected external configuration file examples to use dashes (e.g., fail-on-severity instead of fail_on_severity)
• Removed incorrect note about needing to use underscores in external config files

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[claire153]

Thank you for updating the docs! It does work with dashes, I tested an external config file locally with deny-license