docs: adding document for extending the AsyncAPI specification

pages/docs/concepts/asyncapi-document/_section.md

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+---
+title: 'AsyncAPI Document'
+weight: 50
+---

pages/docs/concepts/asyncapi-document/extending-specification.md

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+---
+title: Extending Specification
+weight: 240
+---
+
+Extending the AsyncAPI specification is a technique that allows developers to include domain-specific or use-case-specific information not supported by the base specification. This extension capability provides customization, enabling APIs to accommodate unique details that would not otherwise fit within the confines of the standard AsyncAPI specification.
+
+The benefits of extending the AsyncAPI specification include enhanced specificity — delivering a clear encoding of necessary API details to applications, and consistency — allowing reusable components across the API. However, extensions should be used sparingly. Since they go beyond the Specification, they may not be universally understood.
+
+## Specification extensions
+
+The AsyncAPI Specification allows the addition of custom properties through patterned fields prefixed with `x-`. This helps you create unique things without causing problems with future updates.
+
+The `x-` prefix is used to define custom properties. These properties are user-defined and won't conflict with future specification versions because any property starting with `x-` is reserved for user definitions.
+
+Here is a diagram explaining specification extensions:
+
+```mermaid
+flowchart TD
+    subgraph "AsyncAPI Specification"
+        A[Specification] --> B[Custom Properties]
+    end
+    style B fill:#47BCEE,stroke:#47BCEE;
+    subgraph "Custom Properties" 
+        B --> C[Patterned fields prefixed with x-]
+        C --> D[User-defined properties]
+        C --> E[Reserved for user definitions]
+    end
+```
+
+The above diagram explains how AsyncAPI specification uses `x-` prefixed fields for custom properties.
+
+Here is a simple example of how to extend the AsyncAPI specification:
+
+```yml
+asyncapi: 3.0.0
+info:
+  title: Cool Example
+  version: 0.1.0
+channels:
+  userSignedUp:
+    address: user/signedup
+    messages:
+      userSignedUp:
+        description: An event describing that a user just signed up.
+        $ref: '#/components/messages/UserSignedUp'
+      x-custom-property: Custom Value        
+```
+
+In the above document, under the `user/signedup` channel, a custom property `x-custom-property` is added. The value assigned to this property is `Custom Value`.
+
+<Remember>
+All available tooling might not support AsyncAPI extensions. The tooling can be extended to understand and handle the added data, especially if the tools are internal or open source.
+</Remember>
+
+## Extending unsupported features
+
+When facing a case where the AsyncAPI specification does not support the required feature, that functionality can be extended using these extensions. If the extended part also benefits other developers, you could contribute to the AsyncAPI specification. This contribution can be made by [creating an issue](https://github.com/asyncapi/website/issues/new?assignees=alequetzalli+-&labels=%F0%9F%93%91+docs&projects=&template=docs.yml&title=%5B%F0%9F%93%91+Docs%5D%3A+) on the AsyncAPI GitHub repository.
+
+Here is a diagram showing details about extending extending unsupported features:
+
+```mermaid
+  graph TD
+  A[Encounter Unsupported Feature]
+  B[Extend using Extensions]
+  C[Beneficial to Others?]
+  D[Contribute to Specification]
+  E[Create an issue on GitHub]
+
+  style A fill:#47BCEE,stroke:#47BCEE;
+
+  A --> B
+  B --> C
+  C -->|Yes| D
+  C -->|No| E
+```
+
+This diagram represents the process of extending AsyncAPI specifications when unsupported features are encountered, with an option to contribute to the specification if it benefits others.
+
+## Using the AsyncAPI studio
+
+When extending the AsyncAPI specification, you can use the[AsyncAPI Studio](https://studio.asyncapi.com/), a recommended dedicated editor for AsyncAPI specifications. The AsyncAPI Studio allows you to create, edit, and visualize specification files. It also performs validation checks to ensure the specifications are correct and renders them in a user-friendly manner.
+
+Here is a diagram showing when to use AsyncAPI studio:
+
+```mermaid
+graph TD
+    A[Extend AsyncAPI Specification] --> B[Use AsyncAPI Studio]
+    B --> C[Create, Edit, Visualize]
+    B --> D[Perform Validation Checks]
+    B --> E[Renders User-Friendly Output]
+
+    style B fill:#47BCEE,stroke:#47BCEE;
+```
+
+ The diagram shows the process of extending the AsyncAPI specification using the recommended AsyncAPI Studio editor.