CI: treat documentation warnings as errors

We do not want to merge documentation that produces Sphinx warnings.

scripts/kernel-doc uses environment variable KDOC_WERROR to determine
if warnings should be treated as errors.

Reported-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>