CR-5987 OPENDJ-1784 Move reference content to reference document

This patch sets up an OpenDJ Reference,
and moves all the reference content out of the OpenDJ Admin Guide. 

This is preparation to move to using generated content
as much as possible for the reference.
This has the side effect of slimming down the OpenDJ Admin Guide,
hopefully making it less imposing.

This is part of OPENDJ-386 Move to single reference document.