mermaid: move mermaid snippet to `scripts/`

README.md

 ![fri](./assets/fri.png)
-
-> generate the diagram
-> ```shell
-> use mmd.nu ["cargo mermaid", "generate mermaid"]
-> cargo mermaid | generate mermaid
-> ```

scripts/mermaid.nu

+#!/usr/bin/env nu
+
+use ../mmd.nu ["cargo mermaid", "generate mermaid"]
+cargo mermaid | generate mermaid