In Icinga 1.x the list of members was set on the group object, in Icinga 2 the list of groups is set on the member objects.
(cherry picked from commit 8d3c8fd107f5c6e0da5551992208213fd4b8a867)
Also apply a new structure.
Deleted the old cluster and client documentation and started "fresh".
This commit also includes various images used in the newly written
"Distributed Monitoring" chapter for better understanding.
fixes#12386
