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/main.py

@@ -133,6 +133,12 @@ def serve(mkdocs_yml: Path) -> None:
         raise click.ClickException(f"mkdocs.yml not found: {mkdocs_yml}")
 
     from mkdocs.commands.serve import serve as mkdocs_serve
+
+    host = "127.0.0.1"
+    port = 8000
+    url = f"http://{host}:{port}/"
+
+    click.echo(f"Serving documentation at {url}")
     mkdocs_serve(config_file=str(mkdocs_yml))