feat(nixos): add CrateDocs MCP service module

- Introduced a NixOS module for configuring the CrateDocs MCP server.
- Added options for enabling the service, setting the port, user, and group.
- Configured a hardened systemd service with security restrictions.

flake.nix

@@ -42,5 +42,6 @@
             default = cratedocs-mcp;
           };
         };
+      nixosModules.default = import ./config.nix;
     };
 }