Reworked doc build to depend on params.xsl file in slides/

Reworked doc build to depend on params.xsl file in slides/
instead of in slides/html. This makes xsl/slides build consistent
with other output formats. With this setup we avoid needing to
deal with potentially having a separate params.xsl file for each
separate subdir of the slides dir -- instead, if when params for
other than HTML output from slides are added, they just all go in
the single slides/param.xsl file.