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

docs/api/server-articles/index.md

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+# Server Articles
+
+Articles related to the EchoHub server built with ASP.NET Core.
+
+## Topics
+
+- Authentication and JWT tokens
+- SignalR hub and real-time messaging
+- File upload and validation
+- Rate limiting configuration
+- Database schema and migrations

docs/api/server-articles/toc.yml

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+- name: Overview
+  href: index.md