Handbook for 3.4

To-dos:

• Replace versions "3.3" with "3.4"
• Check the changelog to see if it affects the handbook
• Go through handbook and see if anything seems worth to be changed
• Add section about the python scripts in bin directory #1019 (closed) !2587 (merged)
• Change package name scrpage2 to newer version scrlayer-scrpage (!2556 (merged))