doc: use system sphinx-build instead of hg clone

Because it is hard to resolve Sphinx dependency by hand. :<
doc: add workaround for slow "sphinx-build gettext"

It should be removed...

See also:

  * https://bitbucket.org/birkenfeld/sphinx/issue/1426/gettext-builder-is-very-slow-during-read
  * https://gist.github.com/knzm/4053935