Add Diátaxis mention in the documentation section

[ezio-melotti]

We have been following the Diátaxis framework for a while now, with good results, however there are no mentions of it in the devguide. We should add a short section about it in https://devguide.python.org/documentation/start-documenting/ with a link to that section in https://devguide.python.org/documentation/style-guide/.