[Docs] Add bart-plugin to docs (#33905)

Signed-off-by: NickLucche <nlucches@redhat.com>

docs/design/plugin_system.md

@@ -45,7 +45,7 @@ Every plugin has three parts:
 
 ## Types of supported plugins
 
-- **General plugins** (with group name `vllm.general_plugins`): The primary use case for these plugins is to register custom, out-of-the-tree models into vLLM. This is done by calling `ModelRegistry.register_model` to register the model inside the plugin function.
+- **General plugins** (with group name `vllm.general_plugins`): The primary use case for these plugins is to register custom, out-of-the-tree models into vLLM. This is done by calling `ModelRegistry.register_model` to register the model inside the plugin function. For an example of an official model plugin, see the [bart-plugin](https://github.com/vllm-project/bart-plugin) which adds support for `BartForConditionalGeneration`.
 
 - **Platform plugins** (with group name `vllm.platform_plugins`): The primary use case for these plugins is to register custom, out-of-the-tree platforms into vLLM. The plugin function should return `None` when the platform is not supported in the current environment, or the platform class's fully qualified name when the platform is supported.