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.

mkdocs.yml

@@ -47,5 +47,12 @@ nav:
   - CLI:
       - Home: openapi_first/cli.md
 
+  - Templates:
+      - Home: openapi_first/templates/index.md
+      - Health App:
+        - Home: openapi_first/templates/health_app/index.md
+        - App: openapi_first/templates/health_app/main.md
+        - Routes: openapi_first/templates/health_app/routes.md
+
   - Errors:
       - Error Hierarchy: openapi_first/errors.md