DOC: Add missing question mark icon
- [x] closes #32469
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

I added the question mark to the `_static` folder. When rebuilding the documentation pages locally using the latest version of the `pydata-bootstrap-sphinx-theme`, the `div.highlight` seems to be styled correctly (`background: white`), removing the _strange green around the code block_.
