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
```

