doc: Disable SmartyPants in generated manual pages
By default, Sphinx tries to pre-process text through SmartyPants,
which attempts to convert ASCII quotes and dashes to Unicode
characters. Unfortunately, this mangles technical text such as command
lines. For instance, this excerpt from notmuch-tag.rst:
Not only are these characters visually confusing and could easily be
mistaken for a single dash, copying and pasting such command lines
into a terminal is doomed to result in incomprehensible error
messages.