💡 Tracking progress

## Feature request
It would be useful to add tracking to next buttons to record tracking progress.

#### What problem does this feature solve?
To know what part the user have already read/studied/learnt

#### What does the proposed API look like?
ExperienceAPI solves this problem: See here:
https://www.ispring.es/blog/xapi-tin-can

#### How should this be implemented in your opinion?
https://www.adaptlearning.org/index.php/get-started/
Above opensource project solves the issue, but looks too heavy for something just documentation oriented.

#### Are you willing to work on this yourself?
I don't know enough the lang to be able to help.

El issue no proporciona archivos, pruebas ni puntos de entrada desde los que empezar. Primero aclara el modelo de tracking y la API; después identifica el código que gestiona los botones de siguiente y define cómo se registra y verifica el progreso de lectura.

Escrito por el modelo de indexación a partir del texto del issue.