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.py

@@ -25,6 +25,11 @@ def _load_template(template: Path | None) -> dict:
 
 
 @click.command("mkdocs")
+@click.option(
+    "--site-name",
+    required=True,
+    help="MkDocs site_name (required)",
+)
 @click.option(
     "--docs-dir",
     type=click.Path(path_type=Path),
@@ -52,6 +57,7 @@ def mkdocs_cmd(
     nav_file: Path,
     template: Path | None,
     out: Path,
+    site_name: str,
 ) -> None:
     """Generate mkdocs.yml from nav spec and template."""
 
@@ -70,6 +76,9 @@ def mkdocs_cmd(
     # Load template (user or built-in)
     data = _load_template(template)
 
+    # Inject site_name
+    data["site_name"] = site_name
+
     # Inject nav
     data["nav"] = nav_block