Combine all workshop MDbooks into one

So we can easier re-use the books and conduct the workshops several times, if need be (e.g. doing a workshop at a company to collect money for the association).

Would be nice, if we had one place for all the workshops we did, or are going to do. Ferrous Systems is doing it like this (look here: https://rust-training.ferrous-systems.com/latest/slides/)