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/nav/mkdocs.py

@@ -19,14 +19,27 @@ class MkDocsNavEmitter:
             entries: List[str] = []
             for p in paths:
                 # Convert filesystem path back to docs-relative path
-                entries.append(self._to_relative(p))
+                rel_path = self._to_relative(p, nav._docs_root)
+                entries.append(rel_path)
             result.append({group: entries})
 
         return result
 
-    def _to_relative(self, path: Path) -> str:
+    def _to_relative(self, path: Path, docs_root: Path | None) -> str:
         """
         Convert a filesystem path to a docs-relative path.
         """
-        # Normalize to POSIX-style for MkDocs
+        if docs_root and path.is_absolute():
+            try:
+                path = path.relative_to(docs_root.resolve())
+            except ValueError:
+                pass
+        elif docs_root:
+            # Handle relative paths (e.g. starting with 'docs/')
+            path_str = path.as_posix()
+            docs_root_str = docs_root.as_posix()
+            if path_str.startswith(docs_root_str + "/"):
+                return path_str[len(docs_root_str) + 1:]
+        
+        # Fallback for other cases
         return path.as_posix().split("/docs/", 1)[-1]