Revive Reference Manual, remove reference pages from User's Guide.

Make version.sgml the central place for updating version numbers in the
documentation.  Document titles now contain the version number of the
release they belong to.

filelist.sgml is the central (and only) place to declare system entities
(i.e., sgml files).  No longer a need to declare them in each document
header.

There is no longer any need to maintain duplicate chapter lists in
postgres.sgml and user/admin/etc.sgml, everything is build from the same
sources.  Some parameter entities allow for different text to be included
when the integrated or a single doc set is generated, which eliminates the
problems that had caused this to fail in the past.