Figure out how to reference the security team in the core governance docs

As requested by fgm in #2457875-76: [policy] Evolving and documenting Drupal core's structure, responsibilities, and decision-making → and #2457875-78: [policy] Evolving and documenting Drupal core's structure, responsibilities, and decision-making → , we should somehow mention the security team in this document. We originally had Security as a topic, but it doesn't map to one of the core gates (perhaps that's a bug on its own) but the request was to still put some documentation in here referencing security in some way.