Added a mechanism for "building" a copy of the current docbook.rng

Added a mechanism for "building" a copy of the current docbook.rng
schema in the releasetools directory. All that this "build" does
is to first make the docbook/relaxng/docbook/docbook.rng file, and
then copy over to the releasetools directory, prepending to that
docbook.rng copy a comment warning that it might not be up-to-date
and that if you really want to make sure you have an up-to-date
copy, you need to build it from the docbook/relaxng/docbook/
sources.
As a result, also checking in a static copy of the generated copy
of the docbook.rng file, and updating the docbook-xsl doc build to
use that instead of using the docbook/relaxng/docbook/docbook.rng
source version.
The sole reason for this change is to make it easier for other
developers to do docbook-xsl release builds -- because the
docbook-xsl release build needs to the docbook.rng file in order
to automatically generate TDG hyperlinks of DocBook element names
encountered in the doc source and in commit messages (which end up
in the generated NEWS.* files and in the release notes.
So this change prevents developers from needing to build the
docbook/relaxng/docbook.
Making builds easier to do is a Good Thing...