Add examples to documentation for DataFrame.swaplevel
- [x ] closes https://github.com/MarcoGorelli/pyladies-sprint-March-2021/issues/13
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry
