docs: build: Don't include stylesheet in intermediate html files generated from RST

docs: build: Don't include stylesheet in intermediate html files generated from RST

'docutils' add a stylesheet to the output html file for direct
consumption. Since we use the html files just as an intermediate step
which is post-processed to add our own stylesheet and drop the docutils
one in the process we can ask 'rst2html' to not add any for an
intermediate file with less garbage.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>