Added back codex-rs/config.md to link to new location (#2778)

Quick fix: point old config.md to new location

Author: ReubenNarad

codex-rs/config.md

@@ -0,0 +1,6 @@
+# Configuration docs moved
+
+This file has moved. Please see the latest configuration documentation here:
+
+- Full config docs: [docs/config.md](../docs/config.md)
+- MCP servers section: [docs/config.md#mcp_servers](../docs/config.md#mcp_servers)