feat: Add documentation files including Code of Conduct, Contributing guidelines, and Security policy; update Markdown linting configuration and scripts

docs/api/core-articles/index.md

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+# Core Articles
+
+Articles related to the EchoHub.Core shared library.
+
+## Topics
+
+- Data models and DTOs
+- SignalR contract interface
+- Validation constants and shared rules

docs/api/core-articles/toc.yml

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+- name: Overview
+  href: index.md