docs: prefer to use rst2html5 instead of rst2html

docs: prefer to use rst2html5 instead of rst2html

Our website is written assuming HTML5 standard & doctype:

  commit b1c81567c7172bc9dcd701cf46ea3f87725d62c7
  Author: Daniel P. Berrangé <berrange@redhat.com>
  Date:   Wed Jul 26 18:01:25 2017 +0100

    docs: switch to using HTML5 doctype declaration

so we want the RST conversion to also use HTML5. Ubuntu 16.04 still
only has the HTML4 generating tools though, so we have that as a
fallback.

Reviewed-by: Andrea Bolognani <abologna@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>