DOC: Improve examples of df.append to better show the ignore_index param
Change index to [0, 2] to stress that ignore_index=True resets the index of both dataframes

- [x] closes #41407 
- [x] tests 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

Test Output: 

```
################################################################################
################################## Validation ##################################
################################################################################

1 Errors found:
	Return value has no description
```


Anything to fix? Sounds not like it's coming from my changes, does it? 

`Sphinx parallel build error:
nbsphinx.NotebookError: UndefinedError in user_guide/style.ipynb:
'nbformat.notebooknode.NotebookNode object' has no attribute 'tags'` No, see #41450  Sorry pressed the wrong button, this was not on purpose Btw, if you merge maser, the CI should be fixed. Another btw, feel free to add the `Return` description, since the validation is complaining that one is missing, and should be very easy to add. thanks @t0asty 