cm-sentinel

Update syntax.md

1

fix: wording was incorrect. Details: Original text stated ```... newlines that come before the action) can be suppressed by placing a minus sign in front of the first set of...

Support for message signatures (RFC 9421)

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### Preflight checklist - [X] I agree to follow this project's [Code of Conduct](https://github.com/dadrus/heimdall/blob/main/CODE_OF_CONDUCT.md). - [X] I have read and am following this repository's [Contribution Guidelines](https://github.com/dadrus/heimdall/blob/main/CONTRIBUTING.md)." - [X] I have...