DOC: add new page 'how to read these doc' in get started section
This page explains the code block are run in notebook

- [x] closes #39130
- [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~
- [x] ~whatsnew entry~
