[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.
 

docs/models/supported_models.md

@@ -174,6 +174,16 @@ class MyConfig(PretrainedConfig):
     - The `list` in the first element of the `tuple` contains the names of the input arguments
     - The `list` in the last element of the `tuple` contains the names of the variables the layer outputs to in your modeling code
 
+### Plugins
+
+Some model architectures are supported via vLLM plugins. These plugins extend vLLM's capabilities through the [plugin system](../design/plugin_system.md).
+
+| Architecture | Models | Plugin Repository |
+|--------------|--------|-------------------|
+| `BartForConditionalGeneration` | BART | [bart-plugin](https://github.com/vllm-project/bart-plugin) |
+
+For other model architectures not natively supported, in particular for Encoder-Decoder models, we recommend following a similar pattern by implementing support through the plugin system.
+
 ## Loading a Model
 
 ### Hugging Face Hub

docs/usage/v1_guide.md

@@ -134,9 +134,12 @@ Please note that prefix caching is not yet supported for any of the above models
 
 #### Encoder-Decoder Models
 
-Whisper is supported. Other models requiring cross-attention between separate
-encoder and decoder (e.g., `BartForConditionalGeneration`,
-`MllamaForConditionalGeneration`) are no longer supported.
+Whisper is supported natively. Other encoder-decoder models are supported via the plugin system:
+
+- **BART**: `BartForConditionalGeneration` is supported via the official [bart-plugin](https://github.com/vllm-project/bart-plugin).
+
+For other encoder-decoder models (e.g., `MllamaForConditionalGeneration`), we recommend
+following a similar pattern by implementing support through the [plugin system](../design/plugin_system.md).
 
 ### Features