fix: make MkDocs generation filesystem-complete and package-aware

- Add filesystem-based module discovery via `discover_module_paths`
- Decouple documentation coverage from Python import behavior
- Ensure GriffeLoader receives a full module list instead of a single root
- Make MkDocs renderer level-agnostic using global package detection
- Emit `index.md` only for true packages, suppress `<package>.md`
- Mirror full dotted module hierarchy into nested docs directories
- Update CLI, exports, and type stubs to expose discovery helper
- Align tests with filesystem-driven module coverage

This fixes missing docs for submodules and removes invalid package `.md` files.

docforge/__init__.py

@@ -6,13 +6,14 @@ All the rendering, exporting, and serving APIs are intentionally private
 until their contracts are finalized.
 """
 
-from .loader import GriffeLoader
+from .loader import GriffeLoader, discover_module_paths
 from .renderers import MkDocsRenderer
 from .cli import main
 from . import model
 
 __all__ = [
     "GriffeLoader",
+    "discover_module_paths",
     "MkDocsRenderer",
     "model",
     "main",