Add Document Types similar to exam types

Currently, it is possible to upload different types of exams and categorise them accordingly. This results in a clear overview for everyone. The user knows exactly what is available and what to expect (past session exams, sample exams, transcripts, etc.). This separation is also ideal for the student associations, because we can directly see , where the need for more material is most important. This kind of separation (currently it can be defined by the "Exam Type" function) would be very desirable to be available for the generic documents as well. The idea would be that one could then separate the documents into sections such as summaries, flashcards, series etc. This would massively improve the overview in this category.