docs, cli: enforce package-bound docs, add template scaffolding, and document CLI usage

- Restrict mkdocstrings generation to real Python packages (require __init__.py)
- Add explicit documentation section for CLI scaffolding and templates
- Generalize CLI to support multiple templates with dynamic discovery
- Package templates correctly for importlib.resources access
- Add fully documented health_app template (app entry point and handlers)
- Fix setuptools package-data configuration for bundled templates

These changes make documentation import-safe, clarify package boundaries,
and provide a deterministic, OpenAPI-first scaffolding workflow via CLI.

openapi_first/templates/health_app/routes.py

@@ -1,2 +1,32 @@
+"""
+OpenAPI operation handlers.
+
+This module defines pure Python callables that implement OpenAPI
+operations for this service. Functions in this module are bound to HTTP
+routes exclusively via OpenAPI ``operationId`` values.
+
+No routing decorators, HTTP metadata, or framework-specific logic
+should appear here. All request/response semantics are defined in the
+OpenAPI specification.
+
+This module serves solely as an operationId namespace.
+"""
+
+
 def get_health():
+    """
+    Health check operation handler.
+
+    This function implements the OpenAPI operation identified by
+    ``operationId: get_health``.
+
+    It contains no routing metadata or framework-specific logic.
+    Request binding, HTTP method, and response semantics are defined
+    exclusively by the OpenAPI specification.
+
+    Returns
+    -------
+    dict
+        A minimal liveness payload indicating service health.
+    """
     return {"status": "ok"}