DOC: Extended the documentation for `DataFrame.sort_values()`
The current docstring for [`DataFrame.sort_values()`](https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.sort_values.html) is lacking clarity, and also contains links that are not converted to hyperlinks. I extended and rephrased part of the documentation for this method. In particular:

- Added further explanation to compare the single-column vs. multi-column sorting,
- Extended explanation for customized sorting via `key` argument,
- Provided a simplified version of `natsort` example, with added explanation.
/preview Website preview of this PR available at: https://pandas.pydata.org/preview/pandas-dev/pandas/57678/ The rendered version of the changes here is available [here](https://pandas.pydata.org/preview/pandas-dev/pandas/57678/docs/reference/api/pandas.DataFrame.sort_values.html), in case it's useful. Thanks @datapythonista for reviewing the update!

Would be happy to fix the issue. Please take a look.

Also thank you for the guidance!
It is indeed my first contribution to a large project, so I have a lot to learn 😄  @datapythonista could you please let me know if any other modification is needed for this PR?

Thanks :) Thanks @aallahyar 