Make documentation listenable as an audio file

[Kaljurand]

Make sure that all the images and code samples in the documentation (https://github.com/Kaljurand/K6nele/blob/gh-pages/docs/et/user_guide.md) have textual labels and that the documentation produces a reasonable output when converted to audio using http://heliraamat.eki.ee/voxpopuli/