doc:conf: use `version (date)` in Sphinx docs

doc:conf: use `version (date)` in Sphinx docs

Since

  commit bed7587fe4910f8e4b892a09675279c5a4ab5484
  Author: W. Trevor King <wking@tremily.us>
  Date:   Thu Oct 25 19:07:09 2012 -0400

    version: use abbreviated SHA instead of  explicit _VERSION in master branch

the version SHA will either be explicit in version(), or version()
will return a release tag.  This makes the long SHA in the Sphinx docs
unnecessary, so we can replace it with the more interesting commit
date.