docs: turn off "smart quotes" in the HTML build

We have long disabled the "html_use_smartypants" option to prevent Sphinx
from mangling "--" sequences (among others).  Unfortunately, Sphinx changed
that option to "smartquotes" in the 1.6.6 release, and seemingly didn't see
fit to warn about the use of the obsolete option, resulting in the
aforementioned mangling returning.  Disable this behavior again and hope
that the option name stays stable for a while.

Reported-by: Zipeng Zhang <zhangzipeng0@foxmail.com>
Link: https://lore.kernel.org/lkml/tencent_CB1A298D31FD221496FF657CD7EF406E6605@qq.com


Reviewed-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>