Created while reviewing MDL-57809

Have been looking for docs related with the progress classes and failed to find any information about them in docs. Only this really old article is around:

https://docs.moodle.org/dev/Progress_Bar

At the same time those classes are using a namespace that does not look correct (\core\progress). We should consider to move them to another place (output maybe?) and kill that case (or alternatively make progress an api, that I don't think it's correct at all).

Ciao