doc: Disable SmartyPants in generated manual pages

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:

  **notmuch** **tag** **--batch** [--input=<*filename*>]

got turned into:

  notmuch tag –batch [–input=<filename>]

That's an en-dash and an em-dash respectively.

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.

* doc/conf.py: Disable SmartyPants.