[AI Bundle] Update documentation for new system_prompt configuration structure

- Update advanced example to use new nested format
- Add comprehensive documentation section explaining both configuration formats
- Document simple string format for basic usage
- Document advanced array format with include_tools option
- Provide clear examples and configuration options reference

Author: OskarStark

src/ai-bundle/doc/index.rst

@@ -71,8 +71,9 @@ Configuration
                 model:
                     class: 'Symfony\AI\Platform\Bridge\OpenAi\Gpt'
                     name: !php/const Symfony\AI\Platform\Bridge\OpenAi\Gpt::GPT_4O_MINI
-                system_prompt: 'You are a helpful assistant that can answer questions.' # The default system prompt of the agent
-                include_tools: true # Include tool definitions at the end of the system prompt
+                system_prompt: # The system prompt configuration
+                    prompt: 'You are a helpful assistant that can answer questions.' # The prompt text
+                    include_tools: true # Include tool definitions at the end of the system prompt
                 tools:
                     # Referencing a service with #[AsTool] attribute
                     - 'Symfony\AI\Agent\Toolbox\Tool\SimilaritySearch'
@@ -144,6 +145,46 @@ Configuration
                 vectorizer: 'ai.vectorizer.mistral_embeddings'
                 store: 'ai.store.memory.research'
 
+System Prompt Configuration
+----------------------------
+
+The ``system_prompt`` configuration supports two formats to provide flexibility for different use cases:
+
+**Simple Format (String)**
+
+For basic usage when you only need to set a prompt text:
+
+.. code-block:: yaml
+
+    ai:
+        agent:
+            my_agent:
+                model:
+                    class: 'Symfony\AI\Platform\Bridge\OpenAi\Gpt'
+                    name: !php/const Symfony\AI\Platform\Bridge\OpenAi\Gpt::GPT_4O_MINI
+                system_prompt: 'You are a helpful assistant.'
+
+**Advanced Format (Array)**
+
+For advanced usage when you need additional configuration like tool inclusion:
+
+.. code-block:: yaml
+
+    ai:
+        agent:
+            my_agent:
+                model:
+                    class: 'Symfony\AI\Platform\Bridge\OpenAi\Gpt'
+                    name: !php/const Symfony\AI\Platform\Bridge\OpenAi\Gpt::GPT_4O_MINI
+                system_prompt:
+                    prompt: 'You are a helpful assistant that can answer questions.'
+                    include_tools: true # Include tool definitions at the end of the system prompt
+
+**Configuration Options**
+
+* ``prompt`` (string): The system prompt text that will be sent to the AI model
+* ``include_tools`` (boolean, default: false): Whether to automatically append tool definitions to the system prompt
+
 Usage
 -----