docs: make sphinx-build be quiet by default

docs: make sphinx-build be quiet by default

The sphinx-build is fairly verbose spitting out pages of output to the
console, which causes errors from other build commands to be scrolled
off the top of the terminal. This can leave the mistaken impression that
the build passed, when in fact there was a failure.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20210812102427.4036399-1-berrange@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>