CI install pymdown-extensions from pip

.gitea/workflows/docs.yaml

@@ -25,7 +25,7 @@ jobs:
       - name: Install mkdocs
         run: |
           pip install --upgrade pip
-          pip install mkdocs mkdocs-material
+          pip install mkdocs mkdocs-material pymdown-extensions
 
       - name: Build
         run: make docs