No keep_files option

I think it would be cool if there was a configuration option such as this one.

- https://github.com/peaceiris/actions-gh-pages?tab=readme-ov-file#%EF%B8%8F-keeping-existing-files-keep_files

Something like...

```yaml
jobs:
deploy:
runs-on: ubuntu-latest
permissions:
contents: write
steps:
- uses: DenverCoder1/doxygen-github-pages-action@v2.0.0
with:
github_token: ${{ secrets.GITHUB_TOKEN }}
keep_files: true # So I will not remove all other files on the branch
```

It is useful because for example in a repository of mine I deploy both [Doxygen documentation](https://github.com/FLAK-ZOSO/Sista/blob/main/.github/workflows/doxygen-gh-pages.yml) and [PyDoc documentation](https://github.com/FLAK-ZOSO/Sista/blob/main/.github/workflows/pydoc-gh-pages.yml) to different URLs on the same GitHub Pages website, and I don't want one to overwrite the other.