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.

docs/openapi_first/templates/health_app/index.md

@@ -0,0 +1,3 @@
+# Health App
+
+::: openapi_first.templates.health_app

docs/openapi_first/templates/health_app/main.md

@@ -0,0 +1,3 @@
+# Main
+
+::: openapi_first.templates.health_app.main

docs/openapi_first/templates/health_app/routes.md

@@ -0,0 +1,3 @@
+# Routes
+
+::: openapi_first.templates.health_app.routes