Fix docs/mkdocs/hooks/remove_announcement.py (#19382)

docs/mkdocs/hooks/remove_announcement.py

 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 import os
+from pathlib import Path
 from typing import Literal
 
 
@@ -8,10 +9,9 @@ def on_startup(command: Literal["build", "gh-deploy", "serve"], dirty: bool):
     # see https://docs.readthedocs.io/en/stable/reference/environment-variables.html # noqa
     if os.getenv('READTHEDOCS_VERSION_TYPE') == "tag":
         # remove the warning banner if the version is a tagged release
-        docs_dir = os.path.dirname(__file__)
-        announcement_path = os.path.join(docs_dir,
-                                         "mkdocs/overrides/main.html")
+        mkdocs_dir = Path(__file__).parent.parent
+        announcement_path = mkdocs_dir / "overrides/main.html"
         # The file might be removed already if the build is triggered multiple
         # times (readthedocs build both HTML and PDF versions separately)
-        if os.path.exists(announcement_path):
+        if announcement_path.exists():
             os.remove(announcement_path)