Add mkdocs-include-markdown-plugin to GitHub Actions workflow

Author: hreikin

.github/workflows/docs.yml

@@ -25,5 +25,5 @@ jobs:
           path: .cache
           restore-keys: |
             mkdocs-material-
-      - run: pip install mkdocs-material mkdocstrings["python"]
+      - run: pip install mkdocs-material mkdocstrings["python"] mkdocs-include-markdown-plugin
       - run: mkdocs gh-deploy --force