DOC: Add example to Series.divmod

- [x] closes #24589
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

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[Current doc](https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.Series.divmod.html); Rendered doc:
![Screenshot 2020-10-17 at 10 44 26 AM](https://user-images.githubusercontent.com/19281800/96326790-3d888d80-1066-11eb-94e7-0a4e0db5d4ba.png)
