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).
Here is a preview of the new examples:
![Screen Shot 2020-11-17 at 17 20 15](https://user-images.githubusercontent.com/25107902/99424702-e80bfe80-28f9-11eb-82e1-9faa14d4ac1b.png)
![Screen Shot 2020-11-17 at 17 20 24](https://user-images.githubusercontent.com/25107902/99424726-ee01df80-28f9-11eb-9a88-1741f8e289aa.png)
 @jreback I have merged with master and now all checks pass. thanks @marktgraham 