DOC: Fixing EX01 - Added examples
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).

Towards https://github.com/pandas-dev/pandas/issues/37875

Note: `pandas.errors.AccessorRegistrationWarning` definition was supposed to be removed.
The warning was [removed](https://github.com/pandas-dev/pandas/pull/18827/commits/a308a2e29286c03c71e3566a881629d996b4821c) but definition left.
(Explanation courtesy of @MarcoGorelli)
Thanks @DeaMariaLeon 