docs: Add detailed docstrings to API models and update OpenAPI spec (#2889)

This PR focuses on improving the developer experience by adding
comprehensive docstrings to the API data models across the Llama Stack.
These docstrings provide detailed explanations for each model and its
fields, making the API easier to understand and use.

**Key changes:**
- **Added Docstrings:** Added reST formatted docstrings to Pydantic
models in the `llama_stack/apis/` directory. This includes models for:
  - Agents (`agents.py`)
  - Benchmarks (`benchmarks.py`)
  - Datasets (`datasets.py`)
  - Inference (`inference.py`)
  - And many other API modules.
- **OpenAPI Spec Update:** Regenerated the OpenAPI specification
(`docs/_static/llama-stack-spec.yaml` and
`docs/_static/llama-stack-spec.html`) to include the new docstrings.
This will be reflected in the API documentation, providing richer
information to users.

**Impact:**
- Developers using the Llama Stack API will have a better understanding
of the data structures.
- The auto-generated API documentation is now more informative.

---------

Co-authored-by: Ashwin Bharambe <ashwin.bharambe@gmail.com>

llama_stack/apis/models/models.py

@@ -23,12 +23,27 @@ class CommonModelFields(BaseModel):
 
 @json_schema_type
 class ModelType(StrEnum):
+    """Enumeration of supported model types in Llama Stack.
+    :cvar llm: Large language model for text generation and completion
+    :cvar embedding: Embedding model for converting text to vector representations
+    """
+
     llm = "llm"
     embedding = "embedding"
 
 
 @json_schema_type
 class Model(CommonModelFields, Resource):
+    """A model resource representing an AI model registered in Llama Stack.
+
+    :param type: The resource type, always 'model' for model resources
+    :param model_type: The type of model (LLM or embedding model)
+    :param metadata: Any additional metadata for this model
+    :param identifier: Unique identifier for this resource in llama stack
+    :param provider_resource_id: Unique identifier for this resource in the provider
+    :param provider_id: ID of the provider that owns this resource
+    """
+
     type: Literal[ResourceType.model] = ResourceType.model
 
     @property