Add dependency to mkdocs workflow

Author: benjimaclellan

.github/workflows/mkdocs_check.yml

@@ -21,5 +21,5 @@ jobs:
           path: .cache
           restore-keys: |
             mkdocs-material-
-      - run: pip install mkdocs-material mkdocstrings mkdocstrings-python pymdown-extensions mknotebooks
+      - run: pip install mkdocs-material mkdocstrings mkdocstrings-python pymdown-extensions mknotebooks mkdocs-minify-plugin
       - run: mkdocs build