Make sure the docs/_include folder exists

Otherwise you get confusing errors in CI

.github/workflows/push.yml

@@ -37,6 +37,7 @@ jobs:
         run: |
           grep -r --files-without-match 'import LeanAPAP' LeanAPAP | sort > 1.txt
           grep -r --files-with-match 'sorry' LeanAPAP | sort > 2.txt
+          mkdir -p docs/_includes
           comm -23 1.txt 2.txt | sed -e 's/^\(.*\)$/- [`\1`](https:\/\/github.com\/YaelDillies\/LeanAPAP\/blob\/master\/\1)/' > docs/_includes/files_to_upstream.md
           rm 1.txt 2.txt