docsite: all links rendered as absolute links in local dev

Created by: bobheadxi

Previously, in local sg run docsite links would be relative, so that one doesn't leave localhost:5080, but they all point to docs.sourcegraph.com now, so navigating to any link will take you out of your local docsite. It seems I can't roll back to an older version of docsite anymore either, since the templating breaks

cc @jhchabran because it might be related to your recent SEO changes