Test if all classifiers have their own docs

[MarcSerraPeralta]

Add a test to check if all the classifiers in the repo have a description file in docs/source/classifiers/

[MarcSerraPeralta]

Researching, I have seen that there is an option with pylint, however I don't know if this can be integrated with pytest. Another option would be to use iq_readout.__all__ and the getattr to examine everything. If the elements have non-empty __doc__ attribute, then it is ok