Add Mermaid

docs/en/test-mermaid.md

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+# Mermaid Test
+
+This is a test diagram.
+
+```mermaid
+graph TD;
+    A[Start] --> B{Is it working?};
+    B -- Yes --> C[Great!];
+    B -- No --> D[Debug more];
+```
+
+End of test.