docs(templates): document CRUD and model CRUD apps and expose them in mkdocs

- Add comprehensive module and function docstrings to crud_app and model_app templates
- Document OpenAPI-first guarantees, non-goals, and usage patterns in templates
- Add template-level __init__.py files with CLI and client usage examples
- Update mkdocs.yml to include CRUD and model-based CRUD template documentation
- Ensure template documentation follows strict package-bound mkdocstrings rules

openapi_first/templates/model_app/models.py

@@ -1,18 +1,50 @@
 """
 Pydantic domain models for the CRUD example.
+
+This module defines Pydantic models that represent the domain entities
+used by the service. These models are referenced by the OpenAPI
+specification for request and response schemas.
+
+The models are declarative and framework-agnostic. They contain no
+persistence logic, validation beyond type constraints, or business
+behavior.
+
+This module is not part of the ``openapi_first`` library API surface.
+It exists solely to support the example application template.
 """
 
 from pydantic import BaseModel
 
 
 class ItemBase(BaseModel):
+    """
+    Base domain model for an item.
+
+    Defines fields common to all item representations.
+    """
+
     name: str
     price: float
 
 
 class ItemCreate(ItemBase):
+    """
+    Domain model for item creation requests.
+
+    This model is used for request bodies when creating new items.
+    It intentionally excludes the ``id`` field, which is assigned
+    by the service.
+    """
+
     pass
 
 
 class Item(ItemBase):
+    """
+    Domain model for a persisted item.
+
+    This model represents the full item state returned in responses,
+    including the server-assigned identifier.
+    """
+
     id: int