build: add Make `doc-only` target

Allows building just docs using existing Node instead of building Node
first.

PR-URL: https://github.com/nodejs/node/pull/3888


Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>