change workflow to clean branch

.github/workflows/doxygen.yml

@@ -9,6 +9,16 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
+        with:
+          fetch-depth: 0
+
+      - name: Clean gh-pages branch
+        run: |
+          git push origin --delete gh-pages
+          git checkout --orphan gh-pages
+          git rm -rf --dry-run .
+          git rm -rf .
+          git push gh-pages
 
       - name: Generate Doxygen
         run: |