Add configuration_wav2vec2.py 
Add configuration_wav2vec2.py to utils/documentation_tests.txt for doctest.

Based on issue: [19487](https://github.com/huggingface/transformers/issues/19487)

@ydshieh could you please check it?
Thanks :)

_The documentation is not available anymore as the PR was closed or merged._<|||||>Thank you very much @ydshieh!!