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/data.py

@@ -2,29 +2,86 @@
 In-memory data store using Pydantic models.
 
 This module is NOT thread-safe and is intended for demos and scaffolds only.
+This module provides a minimal, process-local data store for the
+model-based CRUD example application. It stores and returns domain
+objects defined using Pydantic models and is intended solely for
+demonstration and scaffolding purposes.
+
+The implementation intentionally avoids:
+- persistence
+- concurrency guarantees
+- transactional semantics
+- validation beyond what Pydantic provides
+
+It is not part of the ``openapi_first`` library API surface.
 """
 
 from typing import Dict
 
 from models import Item, ItemCreate
 
+
+# In-memory storage keyed by item ID.
 _items: Dict[int, Item] = {
     1: Item(id=1, name="Apple", price=0.5),
     2: Item(id=2, name="Banana", price=0.3),
 }
 
+# Auto-incrementing identifier.
 _next_id = 3
 
 
 def list_items() -> list[Item]:
+    """
+    Return all items in the data store.
+
+    Returns
+    -------
+    list[Item]
+        A list of item domain objects.
+    """
     return list(_items.values())
 
 
 def get_item(item_id: int) -> Item:
+    """
+    Retrieve a single item by ID.
+
+    Parameters
+    ----------
+    item_id : int
+        Identifier of the item to retrieve.
+
+    Returns
+    -------
+    Item
+        The requested item.
+
+    Raises
+    ------
+    KeyError
+        If the item does not exist.
+    """
     return _items[item_id]
 
 
 def create_item(payload: ItemCreate) -> Item:
+    """
+    Create a new item in the data store.
+
+    A new identifier is assigned automatically. No additional validation
+    is performed beyond Pydantic model validation.
+
+    Parameters
+    ----------
+    payload : ItemCreate
+        Data required to create a new item.
+
+    Returns
+    -------
+    Item
+        The newly created item.
+    """
     global _next_id
     item = Item(id=_next_id, **payload.model_dump())
     _items[_next_id] = item
@@ -33,6 +90,29 @@ def create_item(payload: ItemCreate) -> Item:
 
 
 def update_item(item_id: int, payload: ItemCreate) -> Item:
+    """
+    Replace an existing item in the data store.
+
+    This function performs a full replacement of the stored item.
+    Partial updates are not supported.
+
+    Parameters
+    ----------
+    item_id : int
+        Identifier of the item to update.
+    payload : ItemCreate
+        New item data.
+
+    Returns
+    -------
+    Item
+        The updated item.
+
+    Raises
+    ------
+    KeyError
+        If the item does not exist.
+    """
     if item_id not in _items:
         raise KeyError(item_id)
     item = Item(id=item_id, **payload.model_dump())
@@ -41,4 +121,17 @@ def update_item(item_id: int, payload: ItemCreate) -> Item:
 
 
 def delete_item(item_id: int) -> None:
+    """
+    Remove an item from the data store.
+
+    Parameters
+    ----------
+    item_id : int
+        Identifier of the item to delete.
+
+    Raises
+    ------
+    KeyError
+        If the item does not exist.
+    """
     del _items[item_id]