feat(cli,mkdocs): require site_name, fix nav paths, and echo serve URL

- Require `--site-name` when generating mkdocs.yml to ensure valid configs
- Inject site_name explicitly into generated mkdocs.yml
- Echo MkDocs serve URL (http://127.0.0.1:8000) before starting server
- Fix MkDocs nav emission to correctly resolve docs-relative paths
- Align MkDocs-related optional dependencies with a compatible, pinned set

These changes make MkDocs generation valid by default, improve UX when serving,
and prevent nav path and plugin compatibility issues.

docforge/cli/mkdocs.pyi

@@ -9,6 +9,11 @@ def _load_template(template: Optional[Path]) -> Dict[str, Any]:
 
 
 @click.command("mkdocs")
+@click.option(
+    "--site-name",
+    required=True,
+    help="MkDocs site_name (required)",
+)
 @click.option(
     "--docs-dir",
     type=click.Path(path_type=Path),
@@ -35,5 +40,6 @@ def mkdocs_cmd(
     nav_file: Path,
     template: Optional[Path],
     out: Path,
+    site_name: str,
 ) -> None:
     ...