Replace image with mermaid diagrams

[jongwooo]

Description

This pull request includes an update to the README.md file to replace a static image with a dynamic graph rendered using Mermaid syntax. This change enhances the ease of use, version control and collaboration.

Fixes #29

AS-IS

TO-BE

graph BT
    GatewayClass["GatewayClass<br>foo-com-external-gateway-class"]
    Gateway["Gateway<br>default/demo-gateway-1"]

    subgraph Namespace [Namespace<br>default]
        Gateway["Gateway<br>default/demo-gateway-1"]
        HTTPRoute1["HTTPRoute<br>default/demo-httproute-1"]
        HTTPRoute2["HTTPRoute<br>default/demo-httproute-2"]
        HTTPRoute3["HTTPRoute<br>default/demo-httproute-3"]
        HTTPRoute4["HTTPRoute<br>ns2/demo-httproute-4"]
        Service["Service<br>default/demo-svc"]
    end

    Gateway -->|GatewayClass| GatewayClass
    
    HTTPRoute1 -->|ParentRef| Gateway
    HTTPRoute2 -->|ParentRef| Gateway
    HTTPRoute3 -->|ParentRef| Gateway
    HTTPRoute4 -->|ParentRef| Gateway

    HTTPRoute1 -->|BackendRef| Service
    HTTPRoute2 -->|BackendRef| Service
    HTTPRoute3 -->|BackendRef| Service
    HTTPRoute4 -->|BackendRef| Service

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