DOC: add unqiue value counts example to groupyby guide
Co-authored-by: Abdulelah Almesfer <28743265+abdulelahsm@users.noreply.github.com>
Co-authoerd-by: Abdulellah Alnumay <33042538+Abo7atm@users.noreply.github.com>

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Adding an example to the groupby user guide. The example showcases the usage of ```nunique``` aggregate function. This is from the PyData Global sprint, from [this issue](https://github.com/MarcoGorelli/PyDataGlobal2020-sprint/issues/5). The example is taken from [StackOverflow](https://stackoverflow.com/questions/15411158/pandas-countdistinct-equivalent).