Add docs basic example for RTDCWriter

[paulmueller]

No description provided.

[RaghavaAlajangi]

Under :
https://dclab.readthedocs.io/en/stable/sec_av_rtdc_dataset.html#exporting-data

Add block to provide two examples for an rtdc writer:

1. Simple feature writing (append mode)
2. Advanced feature writing (replace mode)

[paulmueller]

I have just refactored parts of the documentation and noticed that there was already a hidden section on RTDCWriter. It is now here: https://dclab.readthedocs.io/en/latest/sec_av_dc_io.html#writing-to-an-rtdc-file

Feel free to elaborate and expand it.