DOC: add two new examples to the transform docs
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

This PR adds two examples to the transform user guide. The examples apply the transform method to a groupby object. This is from the PyData Global sprint, from [this issue](https://github.com/MarcoGorelli/PyDataGlobal2020-sprint/issues/5). The examples are taken from [here](https://stackoverflow.com/a/37189976/14583166) and [here](https://stackoverflow.com/a/54417351/14583166).