Add some Example for complex data

# Description
Could you please add some description on `https://pywps.readthedocs.io/projects/PyWPS-Demo/en/latest/flask.html` how to deal with complex input and output as in `buffer` for example.
I tried to do the buffering from some local files. But that did not work.

I always get: `No applicable error code, please check error log`. And the Error log, dies not show a reason: `Request rejected due to exception`.

The link I tried was: http://127.0.0.1:5000/wps?service=WPS&request=Execute&version=1.0.0&identifier=buffer&DataInputs=poly_in=@xlink:href=.gml@method=POST@encoding=UTF-8@schema=.xsd;buffer=10&ResponseDocument=buff_out

# Environment

- operating system: Ubuntu 20.04
- Python version: 3.10.6
- PyWPS version: 3.5.2 from pypi
- PyWPS-flask from git