ci(docs): pin mkdocs-material version to fix build errors

.github/workflows/docs.yml

@@ -17,5 +17,5 @@ jobs:
       - uses: 'actions/setup-python@v4'
         with:
           python-version: '3.x'
-      - run: 'pip install mkdocs-material mkdocs-section-index mkdocs-literate-nav'
+      - run: 'pip install mkdocs-material==9.5.18 mkdocs-section-index mkdocs-literate-nav'
       - run: 'mkdocs gh-deploy -b "auto/pages" --force'