Add example for Rosenbrock function

[souravsingh]

Fixes a part of #7168 by adding example for Rosenbrock function.

[ilayn]

numpy is implicitly loaded for examples so you can remove it.

[ilayn]

The doctest actually checks the output of the function hence you have to provide the result. But random vector will keep changing its results. It's better to fix X and typeout the result explicitly

>>> from scipy.optimize import rosen
>>> X = 0.1 * np.arange(10)
>>> rosen(X)
76.56

[ilayn]

I've cleaned up since the example is clear enough. Thanks @souravsingh merging