docs: css: Hide redundant 'Contents:' line in table of contents of RST docs

docs: css: Hide redundant 'Contents:' line in table of contents of RST docs

'docutils' add line saying "Contents:" on top of the table of contents.
We don't have that in other documents nor it's really necessary. Hide it
in the stylesheet as we can select it easily.

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