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/common/responses.py

@@ -13,6 +13,11 @@ from llama_stack.schema_utils import json_schema_type
 
 
 class Order(Enum):
+    """Sort order for paginated responses.
+    :cvar asc: Ascending order
+    :cvar desc: Descending order
+    """
+
     asc = "asc"
     desc = "desc"