docs: add Variables in URL

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

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+---
+title: Server variables
+weight: 275
+---
+
+The server's URL consists of the `host` and `pathname` fields. These values are not always known when you design your system. AsyncAPI enables you to construct dynamic URLs while enhancing the flexibility and maintainability of your AsyncAPI documents. These dynamic values (variables) are placeholders for values you can replace during runtime. You can easily manage multiple endpoints, handling various server configurations and environments.
+
+## Add variables
+
+You can add variables to `server.host` and `server.pathname` by adding a variable between curly braces like `{braces}`. Next, you use `server.variables` to provide definitions of your variables. Finally, leverage `components.serverVariables` to enable reusable variable definitions across multiple servers.
+
+The diagram below describes how to use reusable variables in AsyncAPI.
+
+```mermaid
+graph LR
+  C[servers]
+  F[host]
+  I[protocol]
+  E[pathname]
+  A[components]
+  B[serverVariables]
+  D[variables]
+  C --> F
+  C --> I
+  C --> E
+  C --> D
+  D --> |$ref| B
+  A --> B
+
+  style C fill:#47BCEE,stroke:#000;
+  style D fill:#47BCEE,stroke:#000;
+  style F fill:#47BCEE,stroke:#000;
+  style E fill:#47BCEE,stroke:#000
+```
+
+First, configure the variables in `host` and/or `pathname`. Next, define reusable variables in `components.serverVariables`. Finally, ensure that your `server.variables` from the server reference definitions in the `components.serverVariables` uses `$ref`.
+
+### Servers section
+
+Define the servers section in your AsyncAPI document, including the `host` and `pathname` for your API servers. Use placeholders enclosed in curly braces {} to represent the variables in the server. For example:
+
+```yaml
+servers:
+  production:
+    host: '{subdomain}.example.com:{port}'
+    pathname: '/{version}
+    variables:
+      subdomain:
+        enum:
+          - development
+          - staging
+          - production
+      port:
+        default: '8080'
+      version:
+        enum:
+          - v1
+          - v2
+```
+
+### `serverVariables` section
+
+Define the `components.serverVariables` section in your AsyncAPI document. For each variable used in the server `host` or `pathname`, provide a default value and an optional description to avoid repeating the variable definitions. For example:
+
+```yaml
+components:
+  serverVariables:
+    subdomain:
+      enum:
+        - development
+        - staging
+        - production
+      default: development
+    port:
+      default: '8080'
+    version:
+      enum:
+        - v1
+        - v2
+```
+
+### Define domain and port variables
+
+Use `components.serverVariables` in your server using the [Reference Object](/docs/reference/specification/v3.0.0#referenceObject) to avoid repeating information:
+
+```yml
+    variables:
+      subdomain:
+        $ref: '#/components/serverVariables/subdomain'
+```
+
+Here's the complete AsyncAPI document with the servers' variables for the `host` field:
+
+```yaml
+asyncapi: 3.0.0
+info:
+  title: Example API
+  version: '1.0.0'
+servers:
+  production:
+    host: '{subdomain}.example.com:{port}'
+    pathname: '/{version}'
+    protocol: amqp
+    variables:
+      subdomain:
+        $ref: '#/components/serverVariables/subdomain'
+      port:
+        $ref: '#/components/serverVariables/port'
+      version:
+        $ref: '#/components/serverVariables/version'
+  development:
+    host: '{subdomain}.dev.example.com:{port}'
+    pathname: /v1
+    protocol: amqp
+    variables:
+      subdomain:
+        $ref: '#/components/serverVariables/subdomain'
+      port:
+        $ref: '#/components/serverVariables/port'
+      version:
+        $ref: '#/components/serverVariables/version'
+components:
+  serverVariables:
+    subdomain:
+      enum:
+        - development
+        - staging
+        - production
+      default: development
+    port:
+      default: '8080'
+    version:
+      enum:
+        - v1
+        - v2
+```