DOC: Change user guide style notebook to reST
This way, the content is parsed by Sphinx so that links to the documentation can be automatically inserted.

The downside is, that the downloadable notebook is not as pretty as before (because it contains raw cells with reST code).

Is this something worth the effort?

- [ ] closes #40645
- [ ] tests added / passed
- [ ] 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
