docs: add servers

pages/docs/concepts/asyncapi-document/add-server.md

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+---
+title: Add servers
+weight: 295
+---
+
+A server often acts as a message broker, managing communication between producers and consumers. However, it can represent different concepts too. To understand this better, please first review our [server concepts doc](/docs/concepts/server).
+
+Adding and defining servers is important because it specifies how and where connections are made, enabling the sending and receiving of messages.
+
+Unique keys identify the server and contain information about the server's connection details, such as the host, protocol, and authentication. 
+
+Here is a diagram of server information with selected fields: 
+
+```mermaid
+graph LR
+  C[servers]
+  F[host]
+  I[protocol]
+  E[security]
+  C --> F
+  C --> E
+  C --> I
+```
+
+The server is one of the main sections of the AsyncAPI document next to others like `info`, `channels`, or `operations`.
+
+## Define servers
+
+Include server definitions in your AsyncAPI document to specify which server a channel or operation connects to. Here is an example of defining a server in AsyncAPI:
+
+```yaml
+servers:
+  prod:
+    host: test.mosquitto.org
+    protocol: mqtt
+    description: Test MQTT server
+```
+
+The previous example shows a server setup using the MQTT protocol, where messages are sent to or received from the `test.mosquitto.org` host.
+
+## Server reusability
+
+Add server definitions in a single location, such as `components.servers`, and then refer to them using the `$ref` keyword for easy reuse.
+
+Here's an example of an AsyncAPI document with two servers referenced from the `components` section:
+```yaml
+servers:
+  kafka-test:
+    $ref: '#/components/servers/kafka-test
+  mqtt-test:
+    $ref: '#/components/servers/mqtt-test
+components
+  servers:
+    kafka-test:
+      host: my.kafka.pl
+      protocol: kafka-secure
+      description: Test Kafka server
+    mqtt-test:
+      host: test.mosquitto.org
+      protocol: mqtt
+      description: Test MQTT server
+```
+
+In this example, the main `servers` section lists multiple servers with sharable definitions. You can also store these server definitions separately and use them across various AsyncAPI documents.
+
+## Channel only on specific server
+
+Your AsyncAPI document can outline an application that receives messages on a channel from an MQTT server, while sending messages on a different channel via a Kafka server. This setup requires defining two servers, with each channel being exclusive to one server – one channel is only available on the MQTT server and the other only on the Kafka server. The AsyncAPI document describes this by adding a `servers` array to each channel, containing references to the respective server definitions.
+
+Here's an example of how to add a server reference to a channel:
+
+```yaml
+servers:
+  kafka-test:
+    host: my.kafka.pl
+    protocol: kafka-secure
+    description: Test Kafka server
+  mqtt-test:
+    host: test.mosquitto.org
+    protocol: mqtt
+    description: Test MQTT server
+channels:
+  myChannel:
+    servers:
+      $ref: "#/servers/mqtt-test"
+    message:
+      $ref: '#/components/messages/myMessage'
+```
+
+In this example, the `myChannel` channel is only available on the `mqtt-test` server.