Extend the topic description for the Kafka output (#3048)

This PR extends the **Topics settings** section of the **Kafka output**
docs for both standalone and Fleet-managed Elastic Agents, and:
- adds missing information about setting dynamic topics in the Kafka
output doc for standalone Elastic Agents,
- adds additional information and an example for using the `add_fields`
processor for setting complex, dynamic topic value in both docs,
- fixes a typo and a link text.

Resolves elastic/ingest-docs#1780

Author: vishaangelova

reference/fleet/kafka-output-settings.md

@@ -131,13 +131,27 @@ Use this option to set the Kafka topic for each {{agent}} event.
 
     You can set a static topic, for example `elastic-agent`, or you can choose to set a topic dynamically based on an [Elastic Common Schema (ECS)](ecs://reference/index.md) field. Available fields include:
 
-    * `data_stream_type`
+    * `data_stream.type`
     * `data_stream.dataset`
     * `data_stream.namespace`
     * `@timestamp`
     * `event-dataset`
 
-    You can also set a custom field. This is useful if you’re using the [`add_fields` processor](/reference/fleet/add_fields-processor.md) as part of your {{agent}} input. Otherwise, setting a custom field is not recommended.
+    You can also set a custom field. This is useful if you need to construct a more complex or structured topic name.
+    
+    To set a dynamic topic value for outputting {{agent}} data to Kafka, you can add the [`add_fields` processor](/reference/fleet/add_fields-processor.md) to any integration policies on your {{fleet}}-managed {{agents}}.
+    
+    For example, the following `add_fields` processor creates a dynamic topic value by interpolating multiple [data stream fields](ecs://reference/ecs-data_stream.md):
+
+    ```yaml
+    - add_fields:
+      target: ''
+      fields: 
+        kafka_topic: '${data_stream.type}-${data_stream.dataset}-${data_stream.namespace}' <1>
+    ```
+    1. Depending on the values of the data stream fields, this generates topic names such as `logs-nginx.access-production` or `metrics-system.cpu-staging` as the value of the custom `kafka_topic` field.
+
+    For more information, refer to [](/reference/fleet/agent-processors.md).
 
 
 ### Header settings [_header_settings]

reference/fleet/kafka-output.md

@@ -69,7 +69,7 @@ inputs {
 
 The `kafka` output supports the following settings, grouped by category. Many of these settings have sensible defaults that allow you to run {{agent}} with minimal configuration.
 
-* [Kafka output configuration settings](#output-kafka-commonly-used-settings)
+* [Commonly used settings](#output-kafka-commonly-used-settings)
 * [Authentication settings](#output-kafka-authentication-settings)
 * [Memory queue settings](#output-kafka-memory-queue-settings)
 * [Topics settings](#output-kafka-topics-settings)
@@ -164,6 +164,30 @@ Use these options to set the Kafka topic for each {{agent}} event.
 `topic` $$$kafka-topic-setting$$$
 :   The default Kafka topic used for produced events.
 
+    You can set a static topic, for example `elastic-agent`, or you can choose to set a topic dynamically based on an [Elastic Common Schema (ECS)](ecs://reference/index.md) field. Available fields include:
+
+    * `data_stream.type`
+    * `data_stream.dataset`
+    * `data_stream.namespace`
+    * `@timestamp`
+    * `event-dataset`
+
+    You can also set a custom field. This is useful if you need to construct a more complex or structured topic name. 
+    
+    To set a dynamic topic value for outputting {{agent}} data to Kafka, you can add the [`add_fields` processor](/reference/fleet/add_fields-processor.md) to the input configuration settings of your standalone {{agent}}. 
+    
+    For example, the following `add_fields` processor creates a dynamic topic value by interpolating multiple [data stream fields](ecs://reference/ecs-data_stream.md):
+
+    ```yaml
+    - add_fields:
+      target: ''
+      fields: 
+        kafka_topic: '${data_stream.type}-${data_stream.dataset}-${data_stream.namespace}' <1>
+    ```
+    1. Depending on the values of the data stream fields, this generates topic names such as `logs-nginx.access-production` or `metrics-system.cpu-staging` as the value of the custom `kafka_topic` field.
+
+    For more information, refer to [](/reference/fleet/agent-processors.md).
+
 
 ## Partition settings [output-kafka-partition-settings]