Add mkdocs variable

Author: kevinzjpeng

.github/workflows/build-mkdocs.yml

@@ -40,4 +40,5 @@ jobs:
         run: |
           sed -i "/^extra:/a\  qt_version: ${{ env.QT_VERSION }}" mkdocs.yml
           sed -i "/^extra:/a\  android_version: ${{ env.ANDROID_VERSION }}" mkdocs.yml
+          sed -i "/^extra:/a\  macos_version: ${{ env.MACOS_VERSION }}" mkdocs.yml
       - run: mkdocs gh-deploy --force