docs: open-source edf readers

[pausz]

Alternatives to consider in the future (order in decreasing order of code maturity and complexity):

• https://github.com/mne-tools/mne-python/tree/main/mne/io/edf
• https://pypi.org/project/pyEDFlib/
• https://github.com/holgern/pyedflib
• https://www.teuniz.net/edflib_python/

EDF specfications:

• https://www.edfplus.info/specs/edf.html