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/__init__.py

@@ -0,0 +1,18 @@
+"""
+Application templates for FastAPI OpenAPI First.
+
+This package contains example and scaffolding templates intended to be
+copied into user projects via the ``openapi-first`` CLI.
+
+Templates in this package are:
+- Reference implementations of OpenAPI-first services
+- Not part of the ``openapi_first`` public or internal API
+- Not intended to be imported as runtime dependencies
+
+The presence of this file exists solely to:
+- Mark the directory as an explicit Python package
+- Enable deterministic tooling behavior (documentation, packaging)
+- Avoid accidental traversal of non-package directories
+
+No code in this package should be imported by library consumers.
+"""