Add example section to groupby.rst and one example

following this question on  [StackOverFlow](https://stackoverflow.com/questions/21584434/group-by-value-of-sum-of-columns-with-pandas) and [this issue](https://github.com/pydata/pandas/issues/6288).
I opened an "Examples" section in the doc of `groupby()` and add an example about how a way to regroup columns of a DataFrame according to their sum, and to sum the aggregated ones.