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/loader/__init__.pyi

@@ -1,5 +1,6 @@
-from .griffe_loader import GriffeLoader
+from .griffe_loader import GriffeLoader, discover_module_paths
 
 __all__ = [
-    "GriffeLoader"
+    "GriffeLoader",
+    "discover_module_paths",
 ]