refactor: rename loader/model packages to loaders/models

- Rename docforge.loader → docforge.loaders and docforge.model → docforge.models
- Update all imports, type stubs, CLI, tests, and documentation references
- Align MkDocs navigation and docforge.nav.yml with new package structure
- Adjust module docstrings and comments for consistency with pluralized naming

docforge/models/project.pyi

@@ -0,0 +1,20 @@
+from typing import Dict, Iterable
+
+from docforge.models.module import Module
+
+
+class Project:
+    """Represents a documentation project."""
+
+    name: str
+    modules: Dict[str, Module]
+
+    def __init__(self, name: str) -> None: ...
+
+    def add_module(self, module: Module) -> None: ...
+
+    def get_module(self, path: str) -> Module: ...
+
+    def get_all_modules(self) -> Iterable[Module]: ...
+
+    def get_module_list(self) -> list[str]: ...