docs: add URL variables in AsyncAPI Document

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

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+---
+title: AsyncAPI document
+weight: 50
+---

pages/docs/concepts/asyncapi document/variable-url.md

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+---
+title: Variable URL
+weight: 275
+---
+
+Servers benefit from a defined base URL and rules for URL variables. You can easily manage multiple endpoints, handling various server configurations and environments.
+
+URL variables are placeholders for values you can replace during runtime. AsyncAPI enables you to construct dynamic URLs while enhancing the flexibility and maintainability of your AsyncAPI documents.
+
+## Add dynamic variables
+
+Use the `server.url` and `server.variables` to add variables to a server URL. Leverage `components.serverVariables` to enable reusable variable definitions across multiple servers.
+
+The diagram below describes how to use variable urls in AsyncAPI.
+
+```mermaid
+graph TD
+
+    A[AsyncAPI Server]
+    B[Components]
+    C[Server URL]
+
+style A fill:#47BCEE,stroke:#47BCEE;
+style B fill:#47BCEE,stroke:#47BCEE;
+style C fill:#47BCEE,stroke:#47BCEE;
+
+  B -->|Defines serverVariables| C
+  C -->|Reusable variables| A
+```
+
+### Servers section
+
+Define the servers section in your AsyncAPI document, and include the base URLs for your API servers. Use placeholders enclosed in curly braces {} to represent the variables in the server URL. For example:
+
+```yaml
+servers:
+  production:
+    url: 'https://{subdomain}.example.com:{port}/v1'
+    variables:
+      subdomain:
+        enum:
+          - development
+          - staging
+          - production
+      port:
+        default: '8080'
+```
+
+### `serverVariables` section
+
+Define the components.serverVariables section in your AsyncAPI document. For each variable used in the server URLs, provide a default value and an optional description. This helps you avoid repeating variable defination. For example,
+
+```yaml
+components:
+  serverVariables:
+    subdomain:
+      enum:
+        - development
+        - staging
+        - production
+      default: development
+    port:
+      default: '8080'
+```
+
+### Define domain and port variables
+
+You can use components.serverVariables to avoid repeating variable definations such as domains and ports. To change the values of these variables, update their default values in the components.serverVariables section. Both servers' URLs will reflect the changes.
+
+Here's the complete AsyncAPI document with the server URL variables:
+
+```yaml
+info:
+  title: Example API
+  version: '1.0.0'
+servers:
+  production:
+    host: '{subdomain}.example.com:{port}'
+    pathname: /v1
+    protocol: amqp
+    variables:
+      subdomain:
+        $ref: '#/components/serverVariables/subdomain'
+      port:
+        $ref: '#/components/serverVariables/port'
+  development:
+    host: '{subdomain}.example.com:{port}'
+    pathname: /v1
+    protocol: amqp
+    variables:
+      subdomain:
+        $ref: '#/components/serverVariables/subdomain'
+      port:
+        $ref: '#/components/serverVariables/port'
+components:
+  serverVariables:
+    subdomain:
+      enum:
+        - development
+        - staging
+        - production
+      default: development
+    port:
+      default: '8080'
+```