Split system preferences out of administration into their own chapter

For the system preferences file this means one less level
of headings, making things slightly more readable:

Before:
Administration > Global system preferences > Tab > Pref

After:
Global System preferences > Tab > Pref

source/index.rst

@@ -11,6 +11,7 @@ Koha 18.05 Manual (en)
 
    intro
    installation
+   systempreferences
    administration
    tools
    patrons