Moved xsl stylesheets out of doc/ directory and into docsrc/

Moved xsl stylesheets out of doc/ directory and into docsrc/
directory. mobe css stylesheets into css/ subdirectory within the
doc dir. Reason for the changes is that the docbook-xsl-doc is now
distributed as a separate package, and having the actual doc files
at the root level of the doc directory makes it quicker and easier
for users to get to the docs when they're installed (e.g., in
/usr/share/doc/docbook-xsl/doc on Debian).