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/griffe_loader.pyi

@@ -1,8 +1,16 @@
 from typing import List, Optional
+from pathlib import Path
 
 from docforge.model import Module, Project
 
 
+def discover_module_paths(
+    module_name: str,
+    project_root: Path | None = None,
+) -> List[str]:
+    ...
+
+
 class GriffeLoader:
     """Griffe-based introspection loader.