Link local docs in Fauxton

This is a port from https://github.com/apache/couchdb/pull/232

Already got +1, but let's see what travis says :)

---
Instead of linking to the latest version online, we link to the
docs for the locally installed version. The docs are
generated automatically when running make / make install, if you
have Sphinx installed. For development, the dev-server will just
proxy the request.