Include the documentation's FIR example additional content into notebook and delete rst-file

Meanwhile we have a [Jupyter Notebook for the FIR example](https://pydynamic.readthedocs.io/en/v2.0.0/examples/digital_filtering/Design%20of%20a%20digital%20deconvolution%20filter%20%28FIR%20type%29.html) and an [rst file containing some more formulas](https://pydynamic.readthedocs.io/en/v2.0.0/Deconvolution%20by%20FIR.html). We should combine all info in the notebook and delete the other file.