DOC: Add .to_xyz to io docs
- [x] closes #41839
- [x] tests added / passed
- [x] 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

Screenshot below for updated docs

<img width="558" alt="Screen Shot 2021-06-06 at 7 03 23 PM" src="https://user-images.githubusercontent.com/59709552/120942916-e53bf400-c6f9-11eb-96bc-d16dc852f45c.png">
