meta: add mermaid diagram examples

_meta/Mermaid_diagrams.md

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+```mermaid
+
+%%{init: {'theme':'dark'}}%%
+  graph TD;
+      A-->B;
+      A-->C;
+      B-->D;
+      C-->D;
+```
+
+```mermaid
+%%{init: {'theme':'default'}}%%
+flowchart TD
+    A[Christmas] -->|Get money| B(Go shopping)
+    B --> C{Let me think}
+    C -->|One| D[Laptop]
+    C -->|Two| E[iPhone]
+    C -->|Three| F[fa:fa-car Car]
+```
+
+```mermaid
+%%{init: {'theme':'neutral'}}%%
+erDiagram
+    CUSTOMER }|..|{ DELIVERY-ADDRESS : has
+    CUSTOMER ||--o{ ORDER : places
+    CUSTOMER ||--o{ INVOICE : "liable for"
+    DELIVERY-ADDRESS ||--o{ ORDER : receives
+    INVOICE ||--|{ ORDER : covers
+    ORDER ||--|{ ORDER-ITEM : includes
+    PRODUCT-CATEGORY ||--|{ PRODUCT : contains
+    PRODUCT ||--o{ ORDER-ITEM : "ordered in"
+```
+
+```mermaid
+%%{init: {'theme':'default'}}%%
+classDiagram
+    Animal <|-- Duck
+    Animal <|-- Fish
+    Animal <|-- Zebra
+    Animal : +int age
+    Animal : +String gender
+    Animal: +isMammal()
+    Animal: +mate()
+    class Duck{
+      +String beakColor
+      +swim()
+      +quack()
+    }
+    class Fish{
+      -int sizeInFeet
+      -canEat()
+    }
+    class Zebra{
+      +bool is_wild
+      +run()
+    }
+```