修复 GitHub Actions Pages 部署：setup-python 指定 cache 路径

setup-python 的 cache: pip 默认查找 requirements.txt / pyproject.toml，
我们用的是 requirements-docs.txt，需要显式指定 cache-dependency-path，
否则会报 "No file matched to [**/requirements.txt or **/pyproject.toml]"
导致 build 失败。

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: 2 people

.github/workflows/docs.yml

@@ -26,6 +26,7 @@ jobs:
         with:
           python-version: "3.13"
           cache: pip
+          cache-dependency-path: requirements-docs.txt
       - run: pip install -r requirements-docs.txt
       - run: mkdocs build
       - uses: actions/upload-pages-artifact@v3