Improve standardize_dataframe documentation, behaviour and give examples

[yutanagano]

• Improve io.standardize_dataframe docstring to better explain the function and give examples
• Add an examples notebook following the same examples
• Improve io.standardize_dataframe behaviour by allowing the user to not specify col_mapper if renaming is not needed

[yutanagano]

I've made the necessary changes to the docstring but I can't really see if the docstring will render without errors as I can't seem to trigger the doc generation workflow. We might want to double check before merging

[andim]

Looks good! You should be able to run the docs generation locally by running make html in the docs directory, at least that works on my end.

Thanks a lot for providing this better documentation, Yuta!