Add conversation agent selector documentation (#28077)

Co-authored-by: c0ffeeca7 <38767475+c0ffeeca7@users.noreply.github.com>

source/_docs/blueprint/selectors.markdown

@@ -26,6 +26,7 @@ The following selectors are currently available:
 - [Color temperature selector](#color-temperature-selector)
 - [Config entry selector](#config-entry-selector)
 - [Constant selector](#constant-selector)
+- [Conversation agent selector](#conversation-agent-selector)
 - [Date selector](#date-selector)
 - [Date \& time selector](#date--time-selector)
 - [Device selector](#device-selector)
@@ -365,6 +366,28 @@ boolean:
 
 The output of this selector is the configured value when the toggle is on, it has not output otherwise.
 
+## Conversation agent selector
+
+The conversation agent selector allows picking a conversation agent.
+
+![Screenshot of a conversation agent selector](/images/blueprints/selector-conversation-agent.png)
+
+The selector has 1 option, `language`. This filters the conversation agents shown, depending on the language.
+
+```yaml
+conversation_agent:
+  language: en
+```
+
+{% configuration conversation_agent %}
+language:
+  description: Limits the list of conversation agents to those supporting the specified language.
+  type: string
+  required: false
+{% endconfiguration %}
+
+The output of this selector is the ID of the conversation agent.
+
 ## Date selector
 
 The date selector shows a date input that allows the user to specify a date.