docs: adding document for extending the AsyncAPI specification

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 you to include domain-specific or use case specific information not supported by the base specification or protocol-specific bindings. This extension capability provides customization, enabling APIs to accommodate unique details that would not otherwise fit within the confines of the standard AsyncAPI specification.
+
+## 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.
+
+Extensions can be use in any part of AsyncAPI document.
+
+Here is an example of how to extend the AsyncAPI document with custom property:
+
+```yml
+asyncapi: 3.0.0
+info:
+  title: Cool Example
+  version: 0.1.0
+  x-linkedin: '/company/asyncapi'  
+```
+
+In the above document, you can see an `info` object extended with custom information about LinkedIn account of company owning the application. Custom information is represented by custom property called `x-linkedin`.
+
+<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/spec) on the AsyncAPI GitHub repository. Please get familiar with [contribution guide](https://github.com/asyncapi/spec/blob/master/CONTRIBUTING.md) before making an contribution.