Tutorial: More details about training

Refs https://redmine.teklia.com/issues/7670

• explain why the SQLite export is necessary
• mention that existing manual transcriptions (such as the one available for Pellet) cannot be used directly for training a HTR model, this is why annotation is necessary, to create a link between areas of the image and transcribed text.
• mention 80/10/10 split instead of annotating the whole set of images