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_.

So to understand, you forgot to include this in the initial PR, and it was unrelated to the changes in the theme? With respect to the `svg`, I do think this must have been the case (file was ignored by default, so I had to `add -f`). The background of the code section seems to be a theme-related adjustment which is no longer apparent using the latest version of the theme.