DOC: Improve examples of df.append to better show the ignore_index param

[t0asty]

Change index to [0, 2] to stress that ignore_index=True resets the index of both dataframes

• closes DOC: Unclear first example in pandas.DataFrame.append #41407
• tests passed
• Ensure all linting tests pass, see here for how to run them
• whatsnew entry

Test Output:

################################################################################
################################## Validation ##################################
################################################################################

1 Errors found:
	Return value has no description

[phofl]

Small comment, otherwise lgtm

[phofl]

Thanks very much

[t0asty]

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'

[phofl]

No, see #41450

[phofl]

Sorry pressed the wrong button, this was not on purpose

[datapythonista]

Being honest, I find the original example clearer and more realistic. I think in most cases users will experience appending or concatenating dataframes that both have the default index and start with 0. And ignore_index will be used to get a new default index for both together. That's what the original example illustrates, I find this new one a bit arbitrary.

Not against merging this if other people find it more clear. But if you think the example it's not clear, maybe using meaningful data would make more sense than this change. Like finding two dataframes that the users can relate with and make sense appending. We have some examples with animals, and their number of legs, their maximum speed... And some others that are not just random data.

[datapythonista]

Btw, if you merge maser, the CI should be fixed.

[datapythonista]

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.

[attack68]

very minor comment

[datapythonista]

Thanks @t0asty, lgtm

[jreback]

thanks @t0asty