Some intros for doc chapters.
Started documentation for universal objects

- kernel
Documentation Universal Objects

- terminal object
Documentations for initial object.
Documentation Universal Objects

- cokernel
Bug correction in documentation of initial object.
Merge branch 'master' of ssh://momo.math.rwth-aachen.de/home/sposur/repos.git/CategoriesForHomalg

* 'master' of ssh://momo.math.rwth-aachen.de/home/sposur/repos.git/CategoriesForHomalg:
  Documentation Universal Objects