Disable Sphinx SmartyPants in HTML output

The handling of dashes in particular results in confusing
documentation in a number of instances, since "--" becomes an
en-dash. This disables SmartyPants wholesale, losing smart quotes
along with smart dashes.

With Sphinx 1.6 we could fine-tune the conversion, using the new
smartquotes and smartquotes_action settings.

Signed-off-by: Stephen Kitt <steve@sk2.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>